Limit Question: a better way to solve this limit? where $x to -infty$
$begingroup$
Hey guys so I have this limit:
$$lim_{x to -∞} f(x) = {(x+sqrt{x^2+2x})}$$
I solved it by multiplying numerator and denominator by $$x-sqrt{x^2+2x}$$ and got $-1$ as my answer, but I really don't like how I solved it; any better way to solve it?
limits
$endgroup$
add a comment |
$begingroup$
Hey guys so I have this limit:
$$lim_{x to -∞} f(x) = {(x+sqrt{x^2+2x})}$$
I solved it by multiplying numerator and denominator by $$x-sqrt{x^2+2x}$$ and got $-1$ as my answer, but I really don't like how I solved it; any better way to solve it?
limits
$endgroup$
add a comment |
$begingroup$
Hey guys so I have this limit:
$$lim_{x to -∞} f(x) = {(x+sqrt{x^2+2x})}$$
I solved it by multiplying numerator and denominator by $$x-sqrt{x^2+2x}$$ and got $-1$ as my answer, but I really don't like how I solved it; any better way to solve it?
limits
$endgroup$
Hey guys so I have this limit:
$$lim_{x to -∞} f(x) = {(x+sqrt{x^2+2x})}$$
I solved it by multiplying numerator and denominator by $$x-sqrt{x^2+2x}$$ and got $-1$ as my answer, but I really don't like how I solved it; any better way to solve it?
limits
limits
asked Dec 5 '18 at 22:10
S..S..
545
545
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
You can set $x=-dfrac1tenspace(t>0,,tto 0)$. The expression rewrites as
$$f(x)=-frac1t+sqrt{frac{1mathstrut{}}{t^2}-frac2t}=frac{-1+sqrt{strut 1-2t}}t.$$
This is the rate of variation of the function $sqrt{1-2t};$ from $t=0$, so the limit is
$$bigl(sqrt{1-2t}bigr)'_{t=0}=frac1{2sqrt{1-2t}}cdot(-2)Biggm|_{t=0}=-1.$$
$endgroup$
add a comment |
$begingroup$
You are looking, setting $t=|x|$, at
$$
lim_{ttoinfty} sqrt{t^2-2t} - t
$$
For the sake of it, here is a solution using the Taylor expansion of $sqrt{1+u}$ around $0$: for $t>0$,
$$
sqrt{t^2-2t} - t
= tsqrt{1-frac{2}{t}} - t
= tleft(sqrt{1-frac{2}{t}} - 1right)
= tleft(1-frac{1}{t} + oleft(frac{1}{t}right) - 1right)
= -1+o(1)
$$
showing the limit is indeed $-1$.
$endgroup$
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
add a comment |
$begingroup$
For fun
$t=-x$, let $t >2.$
$(t^2-2t+1-1)^{1/2}-t=$
$((t-1)^2-1)^{1/2} -((t-1)^2)^{1/2}-1;$
Then with $m:=(t-1)^2$.
$(m-1)^{1/2}- m^{1/2} -1$.
$y_m:=(m-1)^{1/2}-m^{1/2}.$
$lim_{m rightarrow infty} y_m=0$.
Can you prove it?
$endgroup$
add a comment |
$begingroup$
I suggest to flip the sign by $y=-xto infty$ then
$$lim_{x to -∞} f(x) = {x+sqrt{x^2+2x}}=lim_{y to infty} f(x) = {-y+sqrt{y^2-2y}}$$
and then
$$sqrt{y^2-2y}-y=left(sqrt{y^2-2y}-yright)cdotfrac{sqrt{y^2-2y}+y}{sqrt{y^2-2y}+y}$$
Edit As another alternative we can use binomial series $tto 0, ,(1+t)^n=1+nt+O(t^2)$, are you aware about that?
We proceed as follows
$$sqrt{y^2-2y}=sqrt{y^2}cdot sqrt{1-2/y}=ycdot(1-2/y)^frac12=ycdot(1-1/y+O(1/y^2))=y-1+O(1/y),$$
then
$$sqrt{y^2-2y}-y=y-1+O(1/y)-y=-1+O(1/y) to -1$$
Indeed the $O(1/y)$ notation collects all the terms in absolute value eventually smaller or equal than $c/y$ with $c$ some positive constant then
$$|O(1/y)|le frac c y to 0 implies O(1/y)to 0$$
$endgroup$
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
|
show 5 more comments
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
You can set $x=-dfrac1tenspace(t>0,,tto 0)$. The expression rewrites as
$$f(x)=-frac1t+sqrt{frac{1mathstrut{}}{t^2}-frac2t}=frac{-1+sqrt{strut 1-2t}}t.$$
This is the rate of variation of the function $sqrt{1-2t};$ from $t=0$, so the limit is
$$bigl(sqrt{1-2t}bigr)'_{t=0}=frac1{2sqrt{1-2t}}cdot(-2)Biggm|_{t=0}=-1.$$
$endgroup$
add a comment |
$begingroup$
You can set $x=-dfrac1tenspace(t>0,,tto 0)$. The expression rewrites as
$$f(x)=-frac1t+sqrt{frac{1mathstrut{}}{t^2}-frac2t}=frac{-1+sqrt{strut 1-2t}}t.$$
This is the rate of variation of the function $sqrt{1-2t};$ from $t=0$, so the limit is
$$bigl(sqrt{1-2t}bigr)'_{t=0}=frac1{2sqrt{1-2t}}cdot(-2)Biggm|_{t=0}=-1.$$
$endgroup$
add a comment |
$begingroup$
You can set $x=-dfrac1tenspace(t>0,,tto 0)$. The expression rewrites as
$$f(x)=-frac1t+sqrt{frac{1mathstrut{}}{t^2}-frac2t}=frac{-1+sqrt{strut 1-2t}}t.$$
This is the rate of variation of the function $sqrt{1-2t};$ from $t=0$, so the limit is
$$bigl(sqrt{1-2t}bigr)'_{t=0}=frac1{2sqrt{1-2t}}cdot(-2)Biggm|_{t=0}=-1.$$
$endgroup$
You can set $x=-dfrac1tenspace(t>0,,tto 0)$. The expression rewrites as
$$f(x)=-frac1t+sqrt{frac{1mathstrut{}}{t^2}-frac2t}=frac{-1+sqrt{strut 1-2t}}t.$$
This is the rate of variation of the function $sqrt{1-2t};$ from $t=0$, so the limit is
$$bigl(sqrt{1-2t}bigr)'_{t=0}=frac1{2sqrt{1-2t}}cdot(-2)Biggm|_{t=0}=-1.$$
answered Dec 5 '18 at 22:29
BernardBernard
119k740113
119k740113
add a comment |
add a comment |
$begingroup$
You are looking, setting $t=|x|$, at
$$
lim_{ttoinfty} sqrt{t^2-2t} - t
$$
For the sake of it, here is a solution using the Taylor expansion of $sqrt{1+u}$ around $0$: for $t>0$,
$$
sqrt{t^2-2t} - t
= tsqrt{1-frac{2}{t}} - t
= tleft(sqrt{1-frac{2}{t}} - 1right)
= tleft(1-frac{1}{t} + oleft(frac{1}{t}right) - 1right)
= -1+o(1)
$$
showing the limit is indeed $-1$.
$endgroup$
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
add a comment |
$begingroup$
You are looking, setting $t=|x|$, at
$$
lim_{ttoinfty} sqrt{t^2-2t} - t
$$
For the sake of it, here is a solution using the Taylor expansion of $sqrt{1+u}$ around $0$: for $t>0$,
$$
sqrt{t^2-2t} - t
= tsqrt{1-frac{2}{t}} - t
= tleft(sqrt{1-frac{2}{t}} - 1right)
= tleft(1-frac{1}{t} + oleft(frac{1}{t}right) - 1right)
= -1+o(1)
$$
showing the limit is indeed $-1$.
$endgroup$
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
add a comment |
$begingroup$
You are looking, setting $t=|x|$, at
$$
lim_{ttoinfty} sqrt{t^2-2t} - t
$$
For the sake of it, here is a solution using the Taylor expansion of $sqrt{1+u}$ around $0$: for $t>0$,
$$
sqrt{t^2-2t} - t
= tsqrt{1-frac{2}{t}} - t
= tleft(sqrt{1-frac{2}{t}} - 1right)
= tleft(1-frac{1}{t} + oleft(frac{1}{t}right) - 1right)
= -1+o(1)
$$
showing the limit is indeed $-1$.
$endgroup$
You are looking, setting $t=|x|$, at
$$
lim_{ttoinfty} sqrt{t^2-2t} - t
$$
For the sake of it, here is a solution using the Taylor expansion of $sqrt{1+u}$ around $0$: for $t>0$,
$$
sqrt{t^2-2t} - t
= tsqrt{1-frac{2}{t}} - t
= tleft(sqrt{1-frac{2}{t}} - 1right)
= tleft(1-frac{1}{t} + oleft(frac{1}{t}right) - 1right)
= -1+o(1)
$$
showing the limit is indeed $-1$.
edited Dec 5 '18 at 22:23
answered Dec 5 '18 at 22:20
Clement C.Clement C.
49.8k33887
49.8k33887
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
add a comment |
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
$begingroup$
I don't know what you expect me to say...
$endgroup$
– Clement C.
Dec 6 '18 at 20:21
add a comment |
$begingroup$
For fun
$t=-x$, let $t >2.$
$(t^2-2t+1-1)^{1/2}-t=$
$((t-1)^2-1)^{1/2} -((t-1)^2)^{1/2}-1;$
Then with $m:=(t-1)^2$.
$(m-1)^{1/2}- m^{1/2} -1$.
$y_m:=(m-1)^{1/2}-m^{1/2}.$
$lim_{m rightarrow infty} y_m=0$.
Can you prove it?
$endgroup$
add a comment |
$begingroup$
For fun
$t=-x$, let $t >2.$
$(t^2-2t+1-1)^{1/2}-t=$
$((t-1)^2-1)^{1/2} -((t-1)^2)^{1/2}-1;$
Then with $m:=(t-1)^2$.
$(m-1)^{1/2}- m^{1/2} -1$.
$y_m:=(m-1)^{1/2}-m^{1/2}.$
$lim_{m rightarrow infty} y_m=0$.
Can you prove it?
$endgroup$
add a comment |
$begingroup$
For fun
$t=-x$, let $t >2.$
$(t^2-2t+1-1)^{1/2}-t=$
$((t-1)^2-1)^{1/2} -((t-1)^2)^{1/2}-1;$
Then with $m:=(t-1)^2$.
$(m-1)^{1/2}- m^{1/2} -1$.
$y_m:=(m-1)^{1/2}-m^{1/2}.$
$lim_{m rightarrow infty} y_m=0$.
Can you prove it?
$endgroup$
For fun
$t=-x$, let $t >2.$
$(t^2-2t+1-1)^{1/2}-t=$
$((t-1)^2-1)^{1/2} -((t-1)^2)^{1/2}-1;$
Then with $m:=(t-1)^2$.
$(m-1)^{1/2}- m^{1/2} -1$.
$y_m:=(m-1)^{1/2}-m^{1/2}.$
$lim_{m rightarrow infty} y_m=0$.
Can you prove it?
answered Dec 5 '18 at 22:58
Peter SzilasPeter Szilas
11.1k2821
11.1k2821
add a comment |
add a comment |
$begingroup$
I suggest to flip the sign by $y=-xto infty$ then
$$lim_{x to -∞} f(x) = {x+sqrt{x^2+2x}}=lim_{y to infty} f(x) = {-y+sqrt{y^2-2y}}$$
and then
$$sqrt{y^2-2y}-y=left(sqrt{y^2-2y}-yright)cdotfrac{sqrt{y^2-2y}+y}{sqrt{y^2-2y}+y}$$
Edit As another alternative we can use binomial series $tto 0, ,(1+t)^n=1+nt+O(t^2)$, are you aware about that?
We proceed as follows
$$sqrt{y^2-2y}=sqrt{y^2}cdot sqrt{1-2/y}=ycdot(1-2/y)^frac12=ycdot(1-1/y+O(1/y^2))=y-1+O(1/y),$$
then
$$sqrt{y^2-2y}-y=y-1+O(1/y)-y=-1+O(1/y) to -1$$
Indeed the $O(1/y)$ notation collects all the terms in absolute value eventually smaller or equal than $c/y$ with $c$ some positive constant then
$$|O(1/y)|le frac c y to 0 implies O(1/y)to 0$$
$endgroup$
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
|
show 5 more comments
$begingroup$
I suggest to flip the sign by $y=-xto infty$ then
$$lim_{x to -∞} f(x) = {x+sqrt{x^2+2x}}=lim_{y to infty} f(x) = {-y+sqrt{y^2-2y}}$$
and then
$$sqrt{y^2-2y}-y=left(sqrt{y^2-2y}-yright)cdotfrac{sqrt{y^2-2y}+y}{sqrt{y^2-2y}+y}$$
Edit As another alternative we can use binomial series $tto 0, ,(1+t)^n=1+nt+O(t^2)$, are you aware about that?
We proceed as follows
$$sqrt{y^2-2y}=sqrt{y^2}cdot sqrt{1-2/y}=ycdot(1-2/y)^frac12=ycdot(1-1/y+O(1/y^2))=y-1+O(1/y),$$
then
$$sqrt{y^2-2y}-y=y-1+O(1/y)-y=-1+O(1/y) to -1$$
Indeed the $O(1/y)$ notation collects all the terms in absolute value eventually smaller or equal than $c/y$ with $c$ some positive constant then
$$|O(1/y)|le frac c y to 0 implies O(1/y)to 0$$
$endgroup$
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
|
show 5 more comments
$begingroup$
I suggest to flip the sign by $y=-xto infty$ then
$$lim_{x to -∞} f(x) = {x+sqrt{x^2+2x}}=lim_{y to infty} f(x) = {-y+sqrt{y^2-2y}}$$
and then
$$sqrt{y^2-2y}-y=left(sqrt{y^2-2y}-yright)cdotfrac{sqrt{y^2-2y}+y}{sqrt{y^2-2y}+y}$$
Edit As another alternative we can use binomial series $tto 0, ,(1+t)^n=1+nt+O(t^2)$, are you aware about that?
We proceed as follows
$$sqrt{y^2-2y}=sqrt{y^2}cdot sqrt{1-2/y}=ycdot(1-2/y)^frac12=ycdot(1-1/y+O(1/y^2))=y-1+O(1/y),$$
then
$$sqrt{y^2-2y}-y=y-1+O(1/y)-y=-1+O(1/y) to -1$$
Indeed the $O(1/y)$ notation collects all the terms in absolute value eventually smaller or equal than $c/y$ with $c$ some positive constant then
$$|O(1/y)|le frac c y to 0 implies O(1/y)to 0$$
$endgroup$
I suggest to flip the sign by $y=-xto infty$ then
$$lim_{x to -∞} f(x) = {x+sqrt{x^2+2x}}=lim_{y to infty} f(x) = {-y+sqrt{y^2-2y}}$$
and then
$$sqrt{y^2-2y}-y=left(sqrt{y^2-2y}-yright)cdotfrac{sqrt{y^2-2y}+y}{sqrt{y^2-2y}+y}$$
Edit As another alternative we can use binomial series $tto 0, ,(1+t)^n=1+nt+O(t^2)$, are you aware about that?
We proceed as follows
$$sqrt{y^2-2y}=sqrt{y^2}cdot sqrt{1-2/y}=ycdot(1-2/y)^frac12=ycdot(1-1/y+O(1/y^2))=y-1+O(1/y),$$
then
$$sqrt{y^2-2y}-y=y-1+O(1/y)-y=-1+O(1/y) to -1$$
Indeed the $O(1/y)$ notation collects all the terms in absolute value eventually smaller or equal than $c/y$ with $c$ some positive constant then
$$|O(1/y)|le frac c y to 0 implies O(1/y)to 0$$
edited Dec 5 '18 at 22:51
answered Dec 5 '18 at 22:13
gimusigimusi
92.8k84494
92.8k84494
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
|
show 5 more comments
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
Not aware of that one, could you explain?
$endgroup$
– S..
Dec 5 '18 at 22:16
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Ok I'll add some details for the second limit.
$endgroup$
– gimusi
Dec 5 '18 at 22:17
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@S.. Do not hesitate to ask for any clarification on that! Maybe the little-o term is not clear to you? Are you more confortable with big-O notation? Let me know about that!
$endgroup$
– gimusi
Dec 5 '18 at 22:21
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@amWhy Thanks for the editing! it's nice now
$endgroup$
– gimusi
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
$begingroup$
@gimusi yeah that term is new for me; would be great if you help me out with that
$endgroup$
– S..
Dec 5 '18 at 22:22
|
show 5 more comments
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