Can we make it equal to x?











up vote
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$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}}$.
This is the famous question. I have to calculate it's value. I found somewhere to the solution to be putting this number equal to a variable $x$. That is,
$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}} = x$.



Then we square both the sides.
$6 +{sqrt{6 +sqrt{6 + ldots}}} = x^2$.



Then we replace the square root thing with $x$.



$6 + x = x^2$ and solve the equation to get the answer as 3.



But I have a doubt that what type of number is this? Is it a real number or not? And if it isn't, how can we perform mathematical operations on it?










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  • 10




    It's the limit of a sequence of real numbers, so, if it exists, it's real.
    – Gerry Myerson
    Nov 14 at 11:49






  • 15




    $$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
    – lab bhattacharjee
    Nov 14 at 11:55






  • 4




    Such expressions are called Nested Radicals
    – Yadati Kiran
    Nov 14 at 11:56






  • 13




    You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
    – user3482749
    Nov 14 at 11:56










  • @GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
    – Eff
    Nov 14 at 11:56

















up vote
17
down vote

favorite
2












$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}}$.
This is the famous question. I have to calculate it's value. I found somewhere to the solution to be putting this number equal to a variable $x$. That is,
$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}} = x$.



Then we square both the sides.
$6 +{sqrt{6 +sqrt{6 + ldots}}} = x^2$.



Then we replace the square root thing with $x$.



$6 + x = x^2$ and solve the equation to get the answer as 3.



But I have a doubt that what type of number is this? Is it a real number or not? And if it isn't, how can we perform mathematical operations on it?










share|cite|improve this question









New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
















  • 10




    It's the limit of a sequence of real numbers, so, if it exists, it's real.
    – Gerry Myerson
    Nov 14 at 11:49






  • 15




    $$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
    – lab bhattacharjee
    Nov 14 at 11:55






  • 4




    Such expressions are called Nested Radicals
    – Yadati Kiran
    Nov 14 at 11:56






  • 13




    You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
    – user3482749
    Nov 14 at 11:56










  • @GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
    – Eff
    Nov 14 at 11:56















up vote
17
down vote

favorite
2









up vote
17
down vote

favorite
2






2





$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}}$.
This is the famous question. I have to calculate it's value. I found somewhere to the solution to be putting this number equal to a variable $x$. That is,
$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}} = x$.



Then we square both the sides.
$6 +{sqrt{6 +sqrt{6 + ldots}}} = x^2$.



Then we replace the square root thing with $x$.



$6 + x = x^2$ and solve the equation to get the answer as 3.



But I have a doubt that what type of number is this? Is it a real number or not? And if it isn't, how can we perform mathematical operations on it?










share|cite|improve this question









New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}}$.
This is the famous question. I have to calculate it's value. I found somewhere to the solution to be putting this number equal to a variable $x$. That is,
$sqrt{6 +sqrt{6 +sqrt{6 + ldots}}} = x$.



Then we square both the sides.
$6 +{sqrt{6 +sqrt{6 + ldots}}} = x^2$.



Then we replace the square root thing with $x$.



$6 + x = x^2$ and solve the equation to get the answer as 3.



But I have a doubt that what type of number is this? Is it a real number or not? And if it isn't, how can we perform mathematical operations on it?







algebra-precalculus nested-radicals infinitesimals






share|cite|improve this question









New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|cite|improve this question









New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|cite|improve this question




share|cite|improve this question








edited Nov 14 at 13:47









Martin Sleziak

44.3k7115266




44.3k7115266






New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









asked Nov 14 at 11:47









Aayush Aggarwal

1105




1105




New contributor




Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Aayush Aggarwal is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.








  • 10




    It's the limit of a sequence of real numbers, so, if it exists, it's real.
    – Gerry Myerson
    Nov 14 at 11:49






  • 15




    $$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
    – lab bhattacharjee
    Nov 14 at 11:55






  • 4




    Such expressions are called Nested Radicals
    – Yadati Kiran
    Nov 14 at 11:56






  • 13




    You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
    – user3482749
    Nov 14 at 11:56










  • @GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
    – Eff
    Nov 14 at 11:56
















  • 10




    It's the limit of a sequence of real numbers, so, if it exists, it's real.
    – Gerry Myerson
    Nov 14 at 11:49






  • 15




    $$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
    – lab bhattacharjee
    Nov 14 at 11:55






  • 4




    Such expressions are called Nested Radicals
    – Yadati Kiran
    Nov 14 at 11:56






  • 13




    You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
    – user3482749
    Nov 14 at 11:56










  • @GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
    – Eff
    Nov 14 at 11:56










10




10




It's the limit of a sequence of real numbers, so, if it exists, it's real.
– Gerry Myerson
Nov 14 at 11:49




It's the limit of a sequence of real numbers, so, if it exists, it's real.
– Gerry Myerson
Nov 14 at 11:49




15




15




$$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
– lab bhattacharjee
Nov 14 at 11:55




$$3=sqrt{6+3}=sqrt{6+sqrt{6+3}}=cdots$$
– lab bhattacharjee
Nov 14 at 11:55




4




4




Such expressions are called Nested Radicals
– Yadati Kiran
Nov 14 at 11:56




Such expressions are called Nested Radicals
– Yadati Kiran
Nov 14 at 11:56




13




13




You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
– user3482749
Nov 14 at 11:56




You have proven that if the sequence converges, then it converges to 3. The next step would be to prove that it actually does converge.
– user3482749
Nov 14 at 11:56












@GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
– Eff
Nov 14 at 11:56






@GerryMyerson True. However, it's worth noting that one can easily make a mistake from similar (but incorrect) reasoning. For example, a limit of a sequence of rational numbers is not necessarily rational (but it is real!).
– Eff
Nov 14 at 11:56












2 Answers
2






active

oldest

votes

















up vote
28
down vote



accepted










Why care must be taken



Equating any such infinite expression to a real number must be done with a hint of caution, because the expression need not have a real value. For example, setting $x = 1-1+1-1+...$ is not correct, since one sees that $1-x = 1+(-1+1-1+1) = x$, so $x = frac 12$ which is absurd from an addition point of view : whenever you truncate the series, it always has value either $1$ or $0$, so where does $frac 12$ comes from? With this logic, it is safe to say $1-1+1-...$ does not evaluate to any finite real number.



However, once we can confirm that the result of such an expression is real and well defined, then we can play with them as we do with real numbers.





Ok, so what about this one?



To confirm that $sqrt{6+sqrt{6+sqrt{6+...}}}$ is a finite real number, we need the language of sequences.



I won't go very far in, but essentially, if we define a sequence of reals by $x_1 = sqrt 6$ and $x_{n+1} = sqrt{6+x_n}$, then $x_ 2 = sqrt{6+sqrt 6}$, $x_3 = sqrt{6+sqrt{6+sqrt 6}}$, and eventually, $x_n$ resembles more and more the expression that we are given to evaluate.



EDITED : I have modified the steps required for showing that $x_n$ is a convergent sequence, to the real number $3$.




  • It is easy to see that $x_n$ is bounded. It is clearly positive for all $n$, and can be shown to be bounded by $3$ above by induction.


  • $a_n$ is an increasing sequence can also be shown easily. Any bounded increasing sequence is convergent.


  • Once convergence is shown, we can then assume that $lim x_n = L$ exists, and then use continuity to take limits on $x_{n+1} = sqrt{6+x_n}$ to see that $L = sqrt{6+L}$. But $L geq 0$ must happen. Thus, $L=3$ is the limit, and hence the value of the expression.





Versatility of sequences



To add to this, sequences also offer versatility. A similar question may be asked : what is:
$$
6+frac{6}{6+frac{6}{6+frac{6}{6+...}}}
$$



What we do here is the same : use the language of sequences, by defining $x_1 = 6$ and $x_{n+1} = 6 + frac{6}{x_n}$. Once again, we can check convergence i.e. that this quantity is a finite real number(But on this occasion, the sequence rather oscillates around the point of convergence before converging). Next, we can use limits to deduce that if $L$ is the value then it satisfies $L = 6+ frac 6L$, which gives one reasonable candidate, $3+sqrt{15}$. So, this expression is actually equal to $3+sqrt{15}$.





It's not easy all the time!



However, the approach using sequences doesn't always give immediate rewards. For example, you could ask for the following :
$$
sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+...}}}}
$$



which also looks like a nested radical.Can we find a sequence which, for large $n$, looks like this expression? Try to write one down, which you can work with.



Anyway, the answer to the above expression is $3$! To see this, we need to use "reverse nesting" :
$$
3 = sqrt{9} = sqrt{1+2cdot 4} = sqrt{1+2sqrt{16}} = sqrt{1+2sqrt{1+15}} \ = sqrt{1+2sqrt{1+3sqrt{25}}} = sqrt{1+2sqrt{1+3sqrt{1+4sqrt{36}}}} \= sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+5sqrt{49}}}}} =...
$$



And just breathe in, breathe out. Ramanujan, class ten I believe.





EDIT



The nested radical method is wrong, from a rigorous point of view, for the reason pointed out in the comments. However, there is a rigorous proof here. The proof by Yiorgos Smyrlis is brilliant.



Note that the "nested radical" method can be used for the earlier problem as well, by $3 = sqrt{6+3} = sqrt{6+sqrt{6+3}} = ...$, but this is unrigorous, only providing intuition. You can try to see if something intuitive can be derived for the continued fraction.






share|cite|improve this answer



















  • 7




    "Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
    – Aaron Stevens
    Nov 14 at 20:12






  • 2




    @AaronStevens $3! neq 3$!
    – Todd Sewell
    Nov 14 at 20:27










  • @ToddSewell Yes I know this.
    – Aaron Stevens
    Nov 14 at 20:29






  • 4




    The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
    – Misha Lavrov
    Nov 14 at 23:59












  • @MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
    – астон вілла олоф мэллбэрг
    Nov 15 at 6:02


















up vote
11
down vote













Let $a_1= sqrt{6}$ and $a_{n+1}=sqrt{6+a_n}$ for $n ge 1$.



It is easy to see, by induction, that $(a_n) $ is increasing and that $0 le a_n le 3$ for all $n$. Hence $(a_n)$ is convergent. If $x$ is the limit of $(a_n)$, then we have $x^2-x-6=0$, thus $x=3$ or $x=-2$. Since all $a_n ge 0$, we have $x=3.$






share|cite|improve this answer





















  • Thanks for making sure this series is convergent.
    – Aayush Aggarwal
    Nov 14 at 12:45











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2 Answers
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active

oldest

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2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
28
down vote



accepted










Why care must be taken



Equating any such infinite expression to a real number must be done with a hint of caution, because the expression need not have a real value. For example, setting $x = 1-1+1-1+...$ is not correct, since one sees that $1-x = 1+(-1+1-1+1) = x$, so $x = frac 12$ which is absurd from an addition point of view : whenever you truncate the series, it always has value either $1$ or $0$, so where does $frac 12$ comes from? With this logic, it is safe to say $1-1+1-...$ does not evaluate to any finite real number.



However, once we can confirm that the result of such an expression is real and well defined, then we can play with them as we do with real numbers.





Ok, so what about this one?



To confirm that $sqrt{6+sqrt{6+sqrt{6+...}}}$ is a finite real number, we need the language of sequences.



I won't go very far in, but essentially, if we define a sequence of reals by $x_1 = sqrt 6$ and $x_{n+1} = sqrt{6+x_n}$, then $x_ 2 = sqrt{6+sqrt 6}$, $x_3 = sqrt{6+sqrt{6+sqrt 6}}$, and eventually, $x_n$ resembles more and more the expression that we are given to evaluate.



EDITED : I have modified the steps required for showing that $x_n$ is a convergent sequence, to the real number $3$.




  • It is easy to see that $x_n$ is bounded. It is clearly positive for all $n$, and can be shown to be bounded by $3$ above by induction.


  • $a_n$ is an increasing sequence can also be shown easily. Any bounded increasing sequence is convergent.


  • Once convergence is shown, we can then assume that $lim x_n = L$ exists, and then use continuity to take limits on $x_{n+1} = sqrt{6+x_n}$ to see that $L = sqrt{6+L}$. But $L geq 0$ must happen. Thus, $L=3$ is the limit, and hence the value of the expression.





Versatility of sequences



To add to this, sequences also offer versatility. A similar question may be asked : what is:
$$
6+frac{6}{6+frac{6}{6+frac{6}{6+...}}}
$$



What we do here is the same : use the language of sequences, by defining $x_1 = 6$ and $x_{n+1} = 6 + frac{6}{x_n}$. Once again, we can check convergence i.e. that this quantity is a finite real number(But on this occasion, the sequence rather oscillates around the point of convergence before converging). Next, we can use limits to deduce that if $L$ is the value then it satisfies $L = 6+ frac 6L$, which gives one reasonable candidate, $3+sqrt{15}$. So, this expression is actually equal to $3+sqrt{15}$.





It's not easy all the time!



However, the approach using sequences doesn't always give immediate rewards. For example, you could ask for the following :
$$
sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+...}}}}
$$



which also looks like a nested radical.Can we find a sequence which, for large $n$, looks like this expression? Try to write one down, which you can work with.



Anyway, the answer to the above expression is $3$! To see this, we need to use "reverse nesting" :
$$
3 = sqrt{9} = sqrt{1+2cdot 4} = sqrt{1+2sqrt{16}} = sqrt{1+2sqrt{1+15}} \ = sqrt{1+2sqrt{1+3sqrt{25}}} = sqrt{1+2sqrt{1+3sqrt{1+4sqrt{36}}}} \= sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+5sqrt{49}}}}} =...
$$



And just breathe in, breathe out. Ramanujan, class ten I believe.





EDIT



The nested radical method is wrong, from a rigorous point of view, for the reason pointed out in the comments. However, there is a rigorous proof here. The proof by Yiorgos Smyrlis is brilliant.



Note that the "nested radical" method can be used for the earlier problem as well, by $3 = sqrt{6+3} = sqrt{6+sqrt{6+3}} = ...$, but this is unrigorous, only providing intuition. You can try to see if something intuitive can be derived for the continued fraction.






share|cite|improve this answer



















  • 7




    "Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
    – Aaron Stevens
    Nov 14 at 20:12






  • 2




    @AaronStevens $3! neq 3$!
    – Todd Sewell
    Nov 14 at 20:27










  • @ToddSewell Yes I know this.
    – Aaron Stevens
    Nov 14 at 20:29






  • 4




    The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
    – Misha Lavrov
    Nov 14 at 23:59












  • @MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
    – астон вілла олоф мэллбэрг
    Nov 15 at 6:02















up vote
28
down vote



accepted










Why care must be taken



Equating any such infinite expression to a real number must be done with a hint of caution, because the expression need not have a real value. For example, setting $x = 1-1+1-1+...$ is not correct, since one sees that $1-x = 1+(-1+1-1+1) = x$, so $x = frac 12$ which is absurd from an addition point of view : whenever you truncate the series, it always has value either $1$ or $0$, so where does $frac 12$ comes from? With this logic, it is safe to say $1-1+1-...$ does not evaluate to any finite real number.



However, once we can confirm that the result of such an expression is real and well defined, then we can play with them as we do with real numbers.





Ok, so what about this one?



To confirm that $sqrt{6+sqrt{6+sqrt{6+...}}}$ is a finite real number, we need the language of sequences.



I won't go very far in, but essentially, if we define a sequence of reals by $x_1 = sqrt 6$ and $x_{n+1} = sqrt{6+x_n}$, then $x_ 2 = sqrt{6+sqrt 6}$, $x_3 = sqrt{6+sqrt{6+sqrt 6}}$, and eventually, $x_n$ resembles more and more the expression that we are given to evaluate.



EDITED : I have modified the steps required for showing that $x_n$ is a convergent sequence, to the real number $3$.




  • It is easy to see that $x_n$ is bounded. It is clearly positive for all $n$, and can be shown to be bounded by $3$ above by induction.


  • $a_n$ is an increasing sequence can also be shown easily. Any bounded increasing sequence is convergent.


  • Once convergence is shown, we can then assume that $lim x_n = L$ exists, and then use continuity to take limits on $x_{n+1} = sqrt{6+x_n}$ to see that $L = sqrt{6+L}$. But $L geq 0$ must happen. Thus, $L=3$ is the limit, and hence the value of the expression.





Versatility of sequences



To add to this, sequences also offer versatility. A similar question may be asked : what is:
$$
6+frac{6}{6+frac{6}{6+frac{6}{6+...}}}
$$



What we do here is the same : use the language of sequences, by defining $x_1 = 6$ and $x_{n+1} = 6 + frac{6}{x_n}$. Once again, we can check convergence i.e. that this quantity is a finite real number(But on this occasion, the sequence rather oscillates around the point of convergence before converging). Next, we can use limits to deduce that if $L$ is the value then it satisfies $L = 6+ frac 6L$, which gives one reasonable candidate, $3+sqrt{15}$. So, this expression is actually equal to $3+sqrt{15}$.





It's not easy all the time!



However, the approach using sequences doesn't always give immediate rewards. For example, you could ask for the following :
$$
sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+...}}}}
$$



which also looks like a nested radical.Can we find a sequence which, for large $n$, looks like this expression? Try to write one down, which you can work with.



Anyway, the answer to the above expression is $3$! To see this, we need to use "reverse nesting" :
$$
3 = sqrt{9} = sqrt{1+2cdot 4} = sqrt{1+2sqrt{16}} = sqrt{1+2sqrt{1+15}} \ = sqrt{1+2sqrt{1+3sqrt{25}}} = sqrt{1+2sqrt{1+3sqrt{1+4sqrt{36}}}} \= sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+5sqrt{49}}}}} =...
$$



And just breathe in, breathe out. Ramanujan, class ten I believe.





EDIT



The nested radical method is wrong, from a rigorous point of view, for the reason pointed out in the comments. However, there is a rigorous proof here. The proof by Yiorgos Smyrlis is brilliant.



Note that the "nested radical" method can be used for the earlier problem as well, by $3 = sqrt{6+3} = sqrt{6+sqrt{6+3}} = ...$, but this is unrigorous, only providing intuition. You can try to see if something intuitive can be derived for the continued fraction.






share|cite|improve this answer



















  • 7




    "Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
    – Aaron Stevens
    Nov 14 at 20:12






  • 2




    @AaronStevens $3! neq 3$!
    – Todd Sewell
    Nov 14 at 20:27










  • @ToddSewell Yes I know this.
    – Aaron Stevens
    Nov 14 at 20:29






  • 4




    The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
    – Misha Lavrov
    Nov 14 at 23:59












  • @MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
    – астон вілла олоф мэллбэрг
    Nov 15 at 6:02













up vote
28
down vote



accepted







up vote
28
down vote



accepted






Why care must be taken



Equating any such infinite expression to a real number must be done with a hint of caution, because the expression need not have a real value. For example, setting $x = 1-1+1-1+...$ is not correct, since one sees that $1-x = 1+(-1+1-1+1) = x$, so $x = frac 12$ which is absurd from an addition point of view : whenever you truncate the series, it always has value either $1$ or $0$, so where does $frac 12$ comes from? With this logic, it is safe to say $1-1+1-...$ does not evaluate to any finite real number.



However, once we can confirm that the result of such an expression is real and well defined, then we can play with them as we do with real numbers.





Ok, so what about this one?



To confirm that $sqrt{6+sqrt{6+sqrt{6+...}}}$ is a finite real number, we need the language of sequences.



I won't go very far in, but essentially, if we define a sequence of reals by $x_1 = sqrt 6$ and $x_{n+1} = sqrt{6+x_n}$, then $x_ 2 = sqrt{6+sqrt 6}$, $x_3 = sqrt{6+sqrt{6+sqrt 6}}$, and eventually, $x_n$ resembles more and more the expression that we are given to evaluate.



EDITED : I have modified the steps required for showing that $x_n$ is a convergent sequence, to the real number $3$.




  • It is easy to see that $x_n$ is bounded. It is clearly positive for all $n$, and can be shown to be bounded by $3$ above by induction.


  • $a_n$ is an increasing sequence can also be shown easily. Any bounded increasing sequence is convergent.


  • Once convergence is shown, we can then assume that $lim x_n = L$ exists, and then use continuity to take limits on $x_{n+1} = sqrt{6+x_n}$ to see that $L = sqrt{6+L}$. But $L geq 0$ must happen. Thus, $L=3$ is the limit, and hence the value of the expression.





Versatility of sequences



To add to this, sequences also offer versatility. A similar question may be asked : what is:
$$
6+frac{6}{6+frac{6}{6+frac{6}{6+...}}}
$$



What we do here is the same : use the language of sequences, by defining $x_1 = 6$ and $x_{n+1} = 6 + frac{6}{x_n}$. Once again, we can check convergence i.e. that this quantity is a finite real number(But on this occasion, the sequence rather oscillates around the point of convergence before converging). Next, we can use limits to deduce that if $L$ is the value then it satisfies $L = 6+ frac 6L$, which gives one reasonable candidate, $3+sqrt{15}$. So, this expression is actually equal to $3+sqrt{15}$.





It's not easy all the time!



However, the approach using sequences doesn't always give immediate rewards. For example, you could ask for the following :
$$
sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+...}}}}
$$



which also looks like a nested radical.Can we find a sequence which, for large $n$, looks like this expression? Try to write one down, which you can work with.



Anyway, the answer to the above expression is $3$! To see this, we need to use "reverse nesting" :
$$
3 = sqrt{9} = sqrt{1+2cdot 4} = sqrt{1+2sqrt{16}} = sqrt{1+2sqrt{1+15}} \ = sqrt{1+2sqrt{1+3sqrt{25}}} = sqrt{1+2sqrt{1+3sqrt{1+4sqrt{36}}}} \= sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+5sqrt{49}}}}} =...
$$



And just breathe in, breathe out. Ramanujan, class ten I believe.





EDIT



The nested radical method is wrong, from a rigorous point of view, for the reason pointed out in the comments. However, there is a rigorous proof here. The proof by Yiorgos Smyrlis is brilliant.



Note that the "nested radical" method can be used for the earlier problem as well, by $3 = sqrt{6+3} = sqrt{6+sqrt{6+3}} = ...$, but this is unrigorous, only providing intuition. You can try to see if something intuitive can be derived for the continued fraction.






share|cite|improve this answer














Why care must be taken



Equating any such infinite expression to a real number must be done with a hint of caution, because the expression need not have a real value. For example, setting $x = 1-1+1-1+...$ is not correct, since one sees that $1-x = 1+(-1+1-1+1) = x$, so $x = frac 12$ which is absurd from an addition point of view : whenever you truncate the series, it always has value either $1$ or $0$, so where does $frac 12$ comes from? With this logic, it is safe to say $1-1+1-...$ does not evaluate to any finite real number.



However, once we can confirm that the result of such an expression is real and well defined, then we can play with them as we do with real numbers.





Ok, so what about this one?



To confirm that $sqrt{6+sqrt{6+sqrt{6+...}}}$ is a finite real number, we need the language of sequences.



I won't go very far in, but essentially, if we define a sequence of reals by $x_1 = sqrt 6$ and $x_{n+1} = sqrt{6+x_n}$, then $x_ 2 = sqrt{6+sqrt 6}$, $x_3 = sqrt{6+sqrt{6+sqrt 6}}$, and eventually, $x_n$ resembles more and more the expression that we are given to evaluate.



EDITED : I have modified the steps required for showing that $x_n$ is a convergent sequence, to the real number $3$.




  • It is easy to see that $x_n$ is bounded. It is clearly positive for all $n$, and can be shown to be bounded by $3$ above by induction.


  • $a_n$ is an increasing sequence can also be shown easily. Any bounded increasing sequence is convergent.


  • Once convergence is shown, we can then assume that $lim x_n = L$ exists, and then use continuity to take limits on $x_{n+1} = sqrt{6+x_n}$ to see that $L = sqrt{6+L}$. But $L geq 0$ must happen. Thus, $L=3$ is the limit, and hence the value of the expression.





Versatility of sequences



To add to this, sequences also offer versatility. A similar question may be asked : what is:
$$
6+frac{6}{6+frac{6}{6+frac{6}{6+...}}}
$$



What we do here is the same : use the language of sequences, by defining $x_1 = 6$ and $x_{n+1} = 6 + frac{6}{x_n}$. Once again, we can check convergence i.e. that this quantity is a finite real number(But on this occasion, the sequence rather oscillates around the point of convergence before converging). Next, we can use limits to deduce that if $L$ is the value then it satisfies $L = 6+ frac 6L$, which gives one reasonable candidate, $3+sqrt{15}$. So, this expression is actually equal to $3+sqrt{15}$.





It's not easy all the time!



However, the approach using sequences doesn't always give immediate rewards. For example, you could ask for the following :
$$
sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+...}}}}
$$



which also looks like a nested radical.Can we find a sequence which, for large $n$, looks like this expression? Try to write one down, which you can work with.



Anyway, the answer to the above expression is $3$! To see this, we need to use "reverse nesting" :
$$
3 = sqrt{9} = sqrt{1+2cdot 4} = sqrt{1+2sqrt{16}} = sqrt{1+2sqrt{1+15}} \ = sqrt{1+2sqrt{1+3sqrt{25}}} = sqrt{1+2sqrt{1+3sqrt{1+4sqrt{36}}}} \= sqrt{1+2sqrt{1+3sqrt{1+4sqrt{1+5sqrt{49}}}}} =...
$$



And just breathe in, breathe out. Ramanujan, class ten I believe.





EDIT



The nested radical method is wrong, from a rigorous point of view, for the reason pointed out in the comments. However, there is a rigorous proof here. The proof by Yiorgos Smyrlis is brilliant.



Note that the "nested radical" method can be used for the earlier problem as well, by $3 = sqrt{6+3} = sqrt{6+sqrt{6+3}} = ...$, but this is unrigorous, only providing intuition. You can try to see if something intuitive can be derived for the continued fraction.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited 2 days ago

























answered Nov 14 at 12:10









астон вілла олоф мэллбэрг

36.2k33375




36.2k33375








  • 7




    "Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
    – Aaron Stevens
    Nov 14 at 20:12






  • 2




    @AaronStevens $3! neq 3$!
    – Todd Sewell
    Nov 14 at 20:27










  • @ToddSewell Yes I know this.
    – Aaron Stevens
    Nov 14 at 20:29






  • 4




    The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
    – Misha Lavrov
    Nov 14 at 23:59












  • @MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
    – астон вілла олоф мэллбэрг
    Nov 15 at 6:02














  • 7




    "Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
    – Aaron Stevens
    Nov 14 at 20:12






  • 2




    @AaronStevens $3! neq 3$!
    – Todd Sewell
    Nov 14 at 20:27










  • @ToddSewell Yes I know this.
    – Aaron Stevens
    Nov 14 at 20:29






  • 4




    The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
    – Misha Lavrov
    Nov 14 at 23:59












  • @MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
    – астон вілла олоф мэллбэрг
    Nov 15 at 6:02








7




7




"Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
– Aaron Stevens
Nov 14 at 20:12




"Anyway, the answer to the above expression is 3!" Thought you meant 3 factorial at first :)
– Aaron Stevens
Nov 14 at 20:12




2




2




@AaronStevens $3! neq 3$!
– Todd Sewell
Nov 14 at 20:27




@AaronStevens $3! neq 3$!
– Todd Sewell
Nov 14 at 20:27












@ToddSewell Yes I know this.
– Aaron Stevens
Nov 14 at 20:29




@ToddSewell Yes I know this.
– Aaron Stevens
Nov 14 at 20:29




4




4




The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
– Misha Lavrov
Nov 14 at 23:59






The "reverse nesting" approach seems suspicious. Any number $x$ has a finite expansion $x = sqrt{1 + 2sqrt{1 + 3sqrt{1 + 4sqrt{1 + 5sqrt{C}}}}}$ for some constant $C$, which we can keep extending arbitrarily as long as we don't care what happens to $C$. In this way you can justify that the infinite radical is equal to anything.
– Misha Lavrov
Nov 14 at 23:59














@MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
– астон вілла олоф мэллбэрг
Nov 15 at 6:02




@MishaLavrov The "nested radical" is wrong, for the reasons you have mentioned. I should add that in the answer above. Indeed, there is a way of working with the above quantity with the help of sequences. I shall add a link to the answer above.
– астон вілла олоф мэллбэрг
Nov 15 at 6:02










up vote
11
down vote













Let $a_1= sqrt{6}$ and $a_{n+1}=sqrt{6+a_n}$ for $n ge 1$.



It is easy to see, by induction, that $(a_n) $ is increasing and that $0 le a_n le 3$ for all $n$. Hence $(a_n)$ is convergent. If $x$ is the limit of $(a_n)$, then we have $x^2-x-6=0$, thus $x=3$ or $x=-2$. Since all $a_n ge 0$, we have $x=3.$






share|cite|improve this answer





















  • Thanks for making sure this series is convergent.
    – Aayush Aggarwal
    Nov 14 at 12:45















up vote
11
down vote













Let $a_1= sqrt{6}$ and $a_{n+1}=sqrt{6+a_n}$ for $n ge 1$.



It is easy to see, by induction, that $(a_n) $ is increasing and that $0 le a_n le 3$ for all $n$. Hence $(a_n)$ is convergent. If $x$ is the limit of $(a_n)$, then we have $x^2-x-6=0$, thus $x=3$ or $x=-2$. Since all $a_n ge 0$, we have $x=3.$






share|cite|improve this answer





















  • Thanks for making sure this series is convergent.
    – Aayush Aggarwal
    Nov 14 at 12:45













up vote
11
down vote










up vote
11
down vote









Let $a_1= sqrt{6}$ and $a_{n+1}=sqrt{6+a_n}$ for $n ge 1$.



It is easy to see, by induction, that $(a_n) $ is increasing and that $0 le a_n le 3$ for all $n$. Hence $(a_n)$ is convergent. If $x$ is the limit of $(a_n)$, then we have $x^2-x-6=0$, thus $x=3$ or $x=-2$. Since all $a_n ge 0$, we have $x=3.$






share|cite|improve this answer












Let $a_1= sqrt{6}$ and $a_{n+1}=sqrt{6+a_n}$ for $n ge 1$.



It is easy to see, by induction, that $(a_n) $ is increasing and that $0 le a_n le 3$ for all $n$. Hence $(a_n)$ is convergent. If $x$ is the limit of $(a_n)$, then we have $x^2-x-6=0$, thus $x=3$ or $x=-2$. Since all $a_n ge 0$, we have $x=3.$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Nov 14 at 12:02









Fred

41.7k1642




41.7k1642












  • Thanks for making sure this series is convergent.
    – Aayush Aggarwal
    Nov 14 at 12:45


















  • Thanks for making sure this series is convergent.
    – Aayush Aggarwal
    Nov 14 at 12:45
















Thanks for making sure this series is convergent.
– Aayush Aggarwal
Nov 14 at 12:45




Thanks for making sure this series is convergent.
– Aayush Aggarwal
Nov 14 at 12:45










Aayush Aggarwal is a new contributor. Be nice, and check out our Code of Conduct.










 

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Aayush Aggarwal is a new contributor. Be nice, and check out our Code of Conduct.















 


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