If $z=4+i sqrt{7}$ then find the value of $z^3 -4z^2 -9z + 91$.












2












$begingroup$


So i was learning complex numbers and i came across this problem. In the solution they have made $z-4=isqrt{7}$ and then they squared the above equation resulting in $z^2 -8z+16=-7$ then they proceeded by sending $-7$ to the LHS resulting in $z^2 -8z+23=0$.



They then divided the original cubic term by the resulting equation from the above steps to get answer as $-1$.
I did not understand why did they do it and how they got $-1$.
The final equation looked like this:
$z^3 -4z^2 -9z + 91=(z^2 -8z+23)(z+4)-1=-1$.



I personally think they used the $x=nq+r$ where $x$ is divided by $n$ resulting in $q$ as the quotient and $r$ as the remainder.
If so then can all the problems like this be solved in the same manner?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Welcome to Maths SX! What don't you understand? The result of the division?
    $endgroup$
    – Bernard
    Dec 9 '18 at 16:57










  • $begingroup$
    I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:02












  • $begingroup$
    The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:05












  • $begingroup$
    Thankyou sir. Also can i solve similar problems in this way?
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:10










  • $begingroup$
    See the answer below, it explains it well.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:11
















2












$begingroup$


So i was learning complex numbers and i came across this problem. In the solution they have made $z-4=isqrt{7}$ and then they squared the above equation resulting in $z^2 -8z+16=-7$ then they proceeded by sending $-7$ to the LHS resulting in $z^2 -8z+23=0$.



They then divided the original cubic term by the resulting equation from the above steps to get answer as $-1$.
I did not understand why did they do it and how they got $-1$.
The final equation looked like this:
$z^3 -4z^2 -9z + 91=(z^2 -8z+23)(z+4)-1=-1$.



I personally think they used the $x=nq+r$ where $x$ is divided by $n$ resulting in $q$ as the quotient and $r$ as the remainder.
If so then can all the problems like this be solved in the same manner?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Welcome to Maths SX! What don't you understand? The result of the division?
    $endgroup$
    – Bernard
    Dec 9 '18 at 16:57










  • $begingroup$
    I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:02












  • $begingroup$
    The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:05












  • $begingroup$
    Thankyou sir. Also can i solve similar problems in this way?
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:10










  • $begingroup$
    See the answer below, it explains it well.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:11














2












2








2





$begingroup$


So i was learning complex numbers and i came across this problem. In the solution they have made $z-4=isqrt{7}$ and then they squared the above equation resulting in $z^2 -8z+16=-7$ then they proceeded by sending $-7$ to the LHS resulting in $z^2 -8z+23=0$.



They then divided the original cubic term by the resulting equation from the above steps to get answer as $-1$.
I did not understand why did they do it and how they got $-1$.
The final equation looked like this:
$z^3 -4z^2 -9z + 91=(z^2 -8z+23)(z+4)-1=-1$.



I personally think they used the $x=nq+r$ where $x$ is divided by $n$ resulting in $q$ as the quotient and $r$ as the remainder.
If so then can all the problems like this be solved in the same manner?










share|cite|improve this question











$endgroup$




So i was learning complex numbers and i came across this problem. In the solution they have made $z-4=isqrt{7}$ and then they squared the above equation resulting in $z^2 -8z+16=-7$ then they proceeded by sending $-7$ to the LHS resulting in $z^2 -8z+23=0$.



They then divided the original cubic term by the resulting equation from the above steps to get answer as $-1$.
I did not understand why did they do it and how they got $-1$.
The final equation looked like this:
$z^3 -4z^2 -9z + 91=(z^2 -8z+23)(z+4)-1=-1$.



I personally think they used the $x=nq+r$ where $x$ is divided by $n$ resulting in $q$ as the quotient and $r$ as the remainder.
If so then can all the problems like this be solved in the same manner?







complex-numbers






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 9 '18 at 17:06







Alex allee

















asked Dec 9 '18 at 16:49









Alex alleeAlex allee

194




194








  • 1




    $begingroup$
    Welcome to Maths SX! What don't you understand? The result of the division?
    $endgroup$
    – Bernard
    Dec 9 '18 at 16:57










  • $begingroup$
    I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:02












  • $begingroup$
    The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:05












  • $begingroup$
    Thankyou sir. Also can i solve similar problems in this way?
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:10










  • $begingroup$
    See the answer below, it explains it well.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:11














  • 1




    $begingroup$
    Welcome to Maths SX! What don't you understand? The result of the division?
    $endgroup$
    – Bernard
    Dec 9 '18 at 16:57










  • $begingroup$
    I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:02












  • $begingroup$
    The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:05












  • $begingroup$
    Thankyou sir. Also can i solve similar problems in this way?
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:10










  • $begingroup$
    See the answer below, it explains it well.
    $endgroup$
    – Winther
    Dec 9 '18 at 17:11








1




1




$begingroup$
Welcome to Maths SX! What don't you understand? The result of the division?
$endgroup$
– Bernard
Dec 9 '18 at 16:57




$begingroup$
Welcome to Maths SX! What don't you understand? The result of the division?
$endgroup$
– Bernard
Dec 9 '18 at 16:57












$begingroup$
I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
$endgroup$
– Alex allee
Dec 9 '18 at 17:02






$begingroup$
I don't understand the method being used. Like is it similar as dividing i^n by i^4 to find the actual value of i^n. Can that same concept even be used in the a complex number equation. In general i need someone to explain it to me like why and what is going on.
$endgroup$
– Alex allee
Dec 9 '18 at 17:02














$begingroup$
The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
$endgroup$
– Winther
Dec 9 '18 at 17:05






$begingroup$
The given $z$ satisfy a second order equation with rational coefficients ($z^2 = Az + B$ for some rational numbers $A,B$). This means that any polynomial expression with rational coefficients can be reduced to the form $Cz+D$ with $C,D$ rational. The way to do this is by polynomial division as you point out. In this particular case $C$ turns out to be $0$ and $D=-1$ so you get the simple answer $-1$.
$endgroup$
– Winther
Dec 9 '18 at 17:05














$begingroup$
Thankyou sir. Also can i solve similar problems in this way?
$endgroup$
– Alex allee
Dec 9 '18 at 17:10




$begingroup$
Thankyou sir. Also can i solve similar problems in this way?
$endgroup$
– Alex allee
Dec 9 '18 at 17:10












$begingroup$
See the answer below, it explains it well.
$endgroup$
– Winther
Dec 9 '18 at 17:11




$begingroup$
See the answer below, it explains it well.
$endgroup$
– Winther
Dec 9 '18 at 17:11










2 Answers
2






active

oldest

votes


















4












$begingroup$

You surely get $(z-4)^2=-7$, which indeed expands to $z^2-8z+23=0$.



Then you can do the long division of $z^3-4z^2-9z+91$ by $z^2-8z+23$, which yields a quotient $z+4$ and remainder $-1$; therefore, for any value of $z$, you have
$$
z^3-4z^2-9z+91=(z^2-8z+23)(z+4)-1
$$

If you substitute the given $z$, then you get
$$
0(4+isqrt{7}+4)-1=-1
$$



Yes, problems of this kind can be treated in this way; if $w$ is any complex number and $g(z)$ is a polynomial such that $g(w)=0$, then for every polynomial $f(z)$ one has
$$
f(z)=q(z)g(z)+r(z)
$$

where the degree of $r$ is less than the degree of $g$, so
$$
f(w)=q(w)g(w)+r(w)=r(w)
$$

and the computation of $r(w)$ is easier.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you sir. 😊
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:11



















1












$begingroup$

Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$






share|cite|improve this answer









$endgroup$













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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    4












    $begingroup$

    You surely get $(z-4)^2=-7$, which indeed expands to $z^2-8z+23=0$.



    Then you can do the long division of $z^3-4z^2-9z+91$ by $z^2-8z+23$, which yields a quotient $z+4$ and remainder $-1$; therefore, for any value of $z$, you have
    $$
    z^3-4z^2-9z+91=(z^2-8z+23)(z+4)-1
    $$

    If you substitute the given $z$, then you get
    $$
    0(4+isqrt{7}+4)-1=-1
    $$



    Yes, problems of this kind can be treated in this way; if $w$ is any complex number and $g(z)$ is a polynomial such that $g(w)=0$, then for every polynomial $f(z)$ one has
    $$
    f(z)=q(z)g(z)+r(z)
    $$

    where the degree of $r$ is less than the degree of $g$, so
    $$
    f(w)=q(w)g(w)+r(w)=r(w)
    $$

    and the computation of $r(w)$ is easier.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Thank you sir. 😊
      $endgroup$
      – Alex allee
      Dec 9 '18 at 17:11
















    4












    $begingroup$

    You surely get $(z-4)^2=-7$, which indeed expands to $z^2-8z+23=0$.



    Then you can do the long division of $z^3-4z^2-9z+91$ by $z^2-8z+23$, which yields a quotient $z+4$ and remainder $-1$; therefore, for any value of $z$, you have
    $$
    z^3-4z^2-9z+91=(z^2-8z+23)(z+4)-1
    $$

    If you substitute the given $z$, then you get
    $$
    0(4+isqrt{7}+4)-1=-1
    $$



    Yes, problems of this kind can be treated in this way; if $w$ is any complex number and $g(z)$ is a polynomial such that $g(w)=0$, then for every polynomial $f(z)$ one has
    $$
    f(z)=q(z)g(z)+r(z)
    $$

    where the degree of $r$ is less than the degree of $g$, so
    $$
    f(w)=q(w)g(w)+r(w)=r(w)
    $$

    and the computation of $r(w)$ is easier.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Thank you sir. 😊
      $endgroup$
      – Alex allee
      Dec 9 '18 at 17:11














    4












    4








    4





    $begingroup$

    You surely get $(z-4)^2=-7$, which indeed expands to $z^2-8z+23=0$.



    Then you can do the long division of $z^3-4z^2-9z+91$ by $z^2-8z+23$, which yields a quotient $z+4$ and remainder $-1$; therefore, for any value of $z$, you have
    $$
    z^3-4z^2-9z+91=(z^2-8z+23)(z+4)-1
    $$

    If you substitute the given $z$, then you get
    $$
    0(4+isqrt{7}+4)-1=-1
    $$



    Yes, problems of this kind can be treated in this way; if $w$ is any complex number and $g(z)$ is a polynomial such that $g(w)=0$, then for every polynomial $f(z)$ one has
    $$
    f(z)=q(z)g(z)+r(z)
    $$

    where the degree of $r$ is less than the degree of $g$, so
    $$
    f(w)=q(w)g(w)+r(w)=r(w)
    $$

    and the computation of $r(w)$ is easier.






    share|cite|improve this answer









    $endgroup$



    You surely get $(z-4)^2=-7$, which indeed expands to $z^2-8z+23=0$.



    Then you can do the long division of $z^3-4z^2-9z+91$ by $z^2-8z+23$, which yields a quotient $z+4$ and remainder $-1$; therefore, for any value of $z$, you have
    $$
    z^3-4z^2-9z+91=(z^2-8z+23)(z+4)-1
    $$

    If you substitute the given $z$, then you get
    $$
    0(4+isqrt{7}+4)-1=-1
    $$



    Yes, problems of this kind can be treated in this way; if $w$ is any complex number and $g(z)$ is a polynomial such that $g(w)=0$, then for every polynomial $f(z)$ one has
    $$
    f(z)=q(z)g(z)+r(z)
    $$

    where the degree of $r$ is less than the degree of $g$, so
    $$
    f(w)=q(w)g(w)+r(w)=r(w)
    $$

    and the computation of $r(w)$ is easier.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Dec 9 '18 at 17:07









    egregegreg

    181k1485203




    181k1485203












    • $begingroup$
      Thank you sir. 😊
      $endgroup$
      – Alex allee
      Dec 9 '18 at 17:11


















    • $begingroup$
      Thank you sir. 😊
      $endgroup$
      – Alex allee
      Dec 9 '18 at 17:11
















    $begingroup$
    Thank you sir. 😊
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:11




    $begingroup$
    Thank you sir. 😊
    $endgroup$
    – Alex allee
    Dec 9 '18 at 17:11











    1












    $begingroup$

    Alternatively, note:
    $$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
    You can replace step-by-step:
    $$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
    =&(z+3)(z-3)(z-4)+55=\
    =&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
    =&(isqrt7+7)(isqrt7-7)+55=\
    =&-7-49+55=\
    =&-1.end{align}$$






    share|cite|improve this answer









    $endgroup$


















      1












      $begingroup$

      Alternatively, note:
      $$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
      You can replace step-by-step:
      $$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
      =&(z+3)(z-3)(z-4)+55=\
      =&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
      =&(isqrt7+7)(isqrt7-7)+55=\
      =&-7-49+55=\
      =&-1.end{align}$$






      share|cite|improve this answer









      $endgroup$
















        1












        1








        1





        $begingroup$

        Alternatively, note:
        $$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
        You can replace step-by-step:
        $$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
        =&(z+3)(z-3)(z-4)+55=\
        =&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
        =&(isqrt7+7)(isqrt7-7)+55=\
        =&-7-49+55=\
        =&-1.end{align}$$






        share|cite|improve this answer









        $endgroup$



        Alternatively, note:
        $$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
        You can replace step-by-step:
        $$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
        =&(z+3)(z-3)(z-4)+55=\
        =&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
        =&(isqrt7+7)(isqrt7-7)+55=\
        =&-7-49+55=\
        =&-1.end{align}$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 9 '18 at 18:15









        farruhotafarruhota

        20k2738




        20k2738






























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