De Moivres Theorem
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This is the questions I got and I'm stuck as to where to proceed after these few steps
complex-numbers
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add a comment |
$begingroup$
This is the questions I got and I'm stuck as to where to proceed after these few steps
complex-numbers
$endgroup$
add a comment |
$begingroup$
This is the questions I got and I'm stuck as to where to proceed after these few steps
complex-numbers
$endgroup$
This is the questions I got and I'm stuck as to where to proceed after these few steps
complex-numbers
complex-numbers
asked Jan 24 at 2:29
user122343user122343
805
805
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4 Answers
4
active
oldest
votes
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For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
$$frac{npi}{8}=theta$$
Now $$4theta = frac{npi}{2}$$
Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
$$x^4-6x^2+1=0$$
How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
$$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
Hope this helps ......
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Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
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I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
add a comment |
$begingroup$
For the first part of the question:
Notice by DMT:
$$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$
Use the binomial expansion to do the rest.
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add a comment |
$begingroup$
For the second part, set $dfrac1{tan4theta }=0$
$implies4theta=(2n+1)dfracpi2$ for some integer $n$
$theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$
$endgroup$
add a comment |
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$$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
$$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.
For example, when $n=2$,
$$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
implying
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
You can similarly derive the result for $n=4$.
Anyway, now you can compute
$$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$
If you divide the numerator and denominator by $cos^4 theta$, this simplifies to
$$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$
Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.
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add a comment |
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
$$frac{npi}{8}=theta$$
Now $$4theta = frac{npi}{2}$$
Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
$$x^4-6x^2+1=0$$
How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
$$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
Hope this helps ......
$endgroup$
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
add a comment |
$begingroup$
For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
$$frac{npi}{8}=theta$$
Now $$4theta = frac{npi}{2}$$
Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
$$x^4-6x^2+1=0$$
How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
$$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
Hope this helps ......
$endgroup$
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
add a comment |
$begingroup$
For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
$$frac{npi}{8}=theta$$
Now $$4theta = frac{npi}{2}$$
Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
$$x^4-6x^2+1=0$$
How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
$$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
Hope this helps ......
$endgroup$
For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
$$frac{npi}{8}=theta$$
Now $$4theta = frac{npi}{2}$$
Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
$$x^4-6x^2+1=0$$
How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
$$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
Hope this helps ......
answered Jan 24 at 4:02
SNEHIL SANYALSNEHIL SANYAL
614110
614110
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
add a comment |
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
Thanks it did help
$endgroup$
– user122343
Jan 25 at 10:26
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
$begingroup$
I am happy to know that, welcome
$endgroup$
– SNEHIL SANYAL
Jan 25 at 13:23
add a comment |
$begingroup$
For the first part of the question:
Notice by DMT:
$$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$
Use the binomial expansion to do the rest.
$endgroup$
add a comment |
$begingroup$
For the first part of the question:
Notice by DMT:
$$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$
Use the binomial expansion to do the rest.
$endgroup$
add a comment |
$begingroup$
For the first part of the question:
Notice by DMT:
$$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$
Use the binomial expansion to do the rest.
$endgroup$
For the first part of the question:
Notice by DMT:
$$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$
Use the binomial expansion to do the rest.
answered Jan 24 at 3:36
Rhys HughesRhys Hughes
6,9571530
6,9571530
add a comment |
add a comment |
$begingroup$
For the second part, set $dfrac1{tan4theta }=0$
$implies4theta=(2n+1)dfracpi2$ for some integer $n$
$theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$
$endgroup$
add a comment |
$begingroup$
For the second part, set $dfrac1{tan4theta }=0$
$implies4theta=(2n+1)dfracpi2$ for some integer $n$
$theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$
$endgroup$
add a comment |
$begingroup$
For the second part, set $dfrac1{tan4theta }=0$
$implies4theta=(2n+1)dfracpi2$ for some integer $n$
$theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$
$endgroup$
For the second part, set $dfrac1{tan4theta }=0$
$implies4theta=(2n+1)dfracpi2$ for some integer $n$
$theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$
answered Jan 24 at 3:55
lab bhattacharjeelab bhattacharjee
226k15157275
226k15157275
add a comment |
add a comment |
$begingroup$
$$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
$$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.
For example, when $n=2$,
$$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
implying
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
You can similarly derive the result for $n=4$.
Anyway, now you can compute
$$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$
If you divide the numerator and denominator by $cos^4 theta$, this simplifies to
$$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$
Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.
$endgroup$
add a comment |
$begingroup$
$$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
$$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.
For example, when $n=2$,
$$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
implying
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
You can similarly derive the result for $n=4$.
Anyway, now you can compute
$$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$
If you divide the numerator and denominator by $cos^4 theta$, this simplifies to
$$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$
Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.
$endgroup$
add a comment |
$begingroup$
$$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
$$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.
For example, when $n=2$,
$$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
implying
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
You can similarly derive the result for $n=4$.
Anyway, now you can compute
$$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$
If you divide the numerator and denominator by $cos^4 theta$, this simplifies to
$$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$
Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.
$endgroup$
$$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
$$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.
For example, when $n=2$,
$$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
implying
$$sin(2theta) = 2sin(theta)cos(theta)$$
$$cos(2theta) = cos^2(theta)-sin^2(theta)$$
You can similarly derive the result for $n=4$.
Anyway, now you can compute
$$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$
If you divide the numerator and denominator by $cos^4 theta$, this simplifies to
$$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$
Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.
answered Jan 24 at 3:53
Aditya DuaAditya Dua
1,15418
1,15418
add a comment |
add a comment |
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