If $z=4+i sqrt{7}$ then find the value of $z^3 -4z^2 -9z + 91$.
$begingroup$
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?
complex-numbers
$endgroup$
add a comment |
$begingroup$
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?
complex-numbers
$endgroup$
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
add a comment |
$begingroup$
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?
complex-numbers
$endgroup$
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
complex-numbers
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
add a comment |
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
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
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.
$endgroup$
$begingroup$
Thank you sir. 😊
$endgroup$
– Alex allee
Dec 9 '18 at 17:11
add a comment |
$begingroup$
Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$
$endgroup$
add a comment |
Your Answer
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2 Answers
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active
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2 Answers
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active
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votes
$begingroup$
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.
$endgroup$
$begingroup$
Thank you sir. 😊
$endgroup$
– Alex allee
Dec 9 '18 at 17:11
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Thank you sir. 😊
$endgroup$
– Alex allee
Dec 9 '18 at 17:11
add a comment |
$begingroup$
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.
$endgroup$
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.
answered Dec 9 '18 at 17:07
egregegreg
181k1485203
181k1485203
$begingroup$
Thank you sir. 😊
$endgroup$
– Alex allee
Dec 9 '18 at 17:11
add a comment |
$begingroup$
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
add a comment |
$begingroup$
Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$
$endgroup$
add a comment |
$begingroup$
Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$
$endgroup$
add a comment |
$begingroup$
Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$
$endgroup$
Alternatively, note:
$$z-4=isqrt7 iff z-3=1+isqrt7 iff z+3=7+isqrt7.$$
You can replace step-by-step:
$$begin{align}z^3 -4z^2 -9z + 91=&z^2(z-4)-9(z-4)+55=\
=&(z+3)(z-3)(z-4)+55=\
=&(isqrt7+7)(1+isqrt7)(isqrt7)+55=\
=&(isqrt7+7)(isqrt7-7)+55=\
=&-7-49+55=\
=&-1.end{align}$$
answered Dec 9 '18 at 18:15
farruhotafarruhota
20k2738
20k2738
add a comment |
add a comment |
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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