Calculate the complex integral
$begingroup$
I have
$$
int{frac{dz}{z^2+9}}
$$
Also I'm given 2 different conditions.
First is $|z|=pi$, second is $|z-2i|=2$.
Okay, so for the integral i have $int{frac{dz}{(z+3i)(z-3i)}}$.
For the first condition, if I draw a circle, then $pi$ will be outside the circle so I can't continue integral calculation using Cauchy's theorem. And for the second one, I'm again not sure how to proceed.
Please shed some light here. Thanks.
complex-integration
$endgroup$
add a comment |
$begingroup$
I have
$$
int{frac{dz}{z^2+9}}
$$
Also I'm given 2 different conditions.
First is $|z|=pi$, second is $|z-2i|=2$.
Okay, so for the integral i have $int{frac{dz}{(z+3i)(z-3i)}}$.
For the first condition, if I draw a circle, then $pi$ will be outside the circle so I can't continue integral calculation using Cauchy's theorem. And for the second one, I'm again not sure how to proceed.
Please shed some light here. Thanks.
complex-integration
$endgroup$
$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39
add a comment |
$begingroup$
I have
$$
int{frac{dz}{z^2+9}}
$$
Also I'm given 2 different conditions.
First is $|z|=pi$, second is $|z-2i|=2$.
Okay, so for the integral i have $int{frac{dz}{(z+3i)(z-3i)}}$.
For the first condition, if I draw a circle, then $pi$ will be outside the circle so I can't continue integral calculation using Cauchy's theorem. And for the second one, I'm again not sure how to proceed.
Please shed some light here. Thanks.
complex-integration
$endgroup$
I have
$$
int{frac{dz}{z^2+9}}
$$
Also I'm given 2 different conditions.
First is $|z|=pi$, second is $|z-2i|=2$.
Okay, so for the integral i have $int{frac{dz}{(z+3i)(z-3i)}}$.
For the first condition, if I draw a circle, then $pi$ will be outside the circle so I can't continue integral calculation using Cauchy's theorem. And for the second one, I'm again not sure how to proceed.
Please shed some light here. Thanks.
complex-integration
complex-integration
asked Dec 18 '18 at 4:33
user3132457user3132457
1598
1598
$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39
add a comment |
$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39
$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
For $mid zmid=pi$ the poles are both inside the circle. For $mid z-2imid =2$ only $3i$ is within the region.
Now use the residue theorem. That is, $f(z)=frac1{z^2+9}=frac1{6i}(frac1{z-3i}-frac1{z+3i})$. The residue at $3i$ is $frac1{6i}$. So the second integral is $2pi icdotfrac1{6i}=frac{pi}3$.
As for the first, we get the difference of the integrals over two smaller contours, both of which are $frac{pi}3$, so $0$.
$endgroup$
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
add a comment |
$begingroup$
For $|z|=pi$ the residue theorem implies that $int frac{1}{(z+3i)(z-3i)}dz = 2pi i [(Res f, 3i) + (Res f, -3i)]$ where $f(z) = frac{1}{z^2+9}$. The other case is similar but only one of the poles lies in the interior of the circle.
$endgroup$
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
For $mid zmid=pi$ the poles are both inside the circle. For $mid z-2imid =2$ only $3i$ is within the region.
Now use the residue theorem. That is, $f(z)=frac1{z^2+9}=frac1{6i}(frac1{z-3i}-frac1{z+3i})$. The residue at $3i$ is $frac1{6i}$. So the second integral is $2pi icdotfrac1{6i}=frac{pi}3$.
As for the first, we get the difference of the integrals over two smaller contours, both of which are $frac{pi}3$, so $0$.
$endgroup$
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
add a comment |
$begingroup$
For $mid zmid=pi$ the poles are both inside the circle. For $mid z-2imid =2$ only $3i$ is within the region.
Now use the residue theorem. That is, $f(z)=frac1{z^2+9}=frac1{6i}(frac1{z-3i}-frac1{z+3i})$. The residue at $3i$ is $frac1{6i}$. So the second integral is $2pi icdotfrac1{6i}=frac{pi}3$.
As for the first, we get the difference of the integrals over two smaller contours, both of which are $frac{pi}3$, so $0$.
$endgroup$
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
add a comment |
$begingroup$
For $mid zmid=pi$ the poles are both inside the circle. For $mid z-2imid =2$ only $3i$ is within the region.
Now use the residue theorem. That is, $f(z)=frac1{z^2+9}=frac1{6i}(frac1{z-3i}-frac1{z+3i})$. The residue at $3i$ is $frac1{6i}$. So the second integral is $2pi icdotfrac1{6i}=frac{pi}3$.
As for the first, we get the difference of the integrals over two smaller contours, both of which are $frac{pi}3$, so $0$.
$endgroup$
For $mid zmid=pi$ the poles are both inside the circle. For $mid z-2imid =2$ only $3i$ is within the region.
Now use the residue theorem. That is, $f(z)=frac1{z^2+9}=frac1{6i}(frac1{z-3i}-frac1{z+3i})$. The residue at $3i$ is $frac1{6i}$. So the second integral is $2pi icdotfrac1{6i}=frac{pi}3$.
As for the first, we get the difference of the integrals over two smaller contours, both of which are $frac{pi}3$, so $0$.
edited Dec 18 '18 at 5:19
answered Dec 18 '18 at 5:14
Chris CusterChris Custer
13.7k3827
13.7k3827
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
add a comment |
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
$f(z)={frac{1}{z^2+9}}$
$endgroup$
– user3132457
Dec 18 '18 at 5:18
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Right you are. Fixed it.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:21
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
Thanks, and how did you expand $f(z)$?
$endgroup$
– user3132457
Dec 18 '18 at 5:22
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
$begingroup$
I factored the denominator. Then did sort of a partial fractions.
$endgroup$
– Chris Custer
Dec 18 '18 at 5:26
add a comment |
$begingroup$
For $|z|=pi$ the residue theorem implies that $int frac{1}{(z+3i)(z-3i)}dz = 2pi i [(Res f, 3i) + (Res f, -3i)]$ where $f(z) = frac{1}{z^2+9}$. The other case is similar but only one of the poles lies in the interior of the circle.
$endgroup$
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
add a comment |
$begingroup$
For $|z|=pi$ the residue theorem implies that $int frac{1}{(z+3i)(z-3i)}dz = 2pi i [(Res f, 3i) + (Res f, -3i)]$ where $f(z) = frac{1}{z^2+9}$. The other case is similar but only one of the poles lies in the interior of the circle.
$endgroup$
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
add a comment |
$begingroup$
For $|z|=pi$ the residue theorem implies that $int frac{1}{(z+3i)(z-3i)}dz = 2pi i [(Res f, 3i) + (Res f, -3i)]$ where $f(z) = frac{1}{z^2+9}$. The other case is similar but only one of the poles lies in the interior of the circle.
$endgroup$
For $|z|=pi$ the residue theorem implies that $int frac{1}{(z+3i)(z-3i)}dz = 2pi i [(Res f, 3i) + (Res f, -3i)]$ where $f(z) = frac{1}{z^2+9}$. The other case is similar but only one of the poles lies in the interior of the circle.
answered Dec 18 '18 at 4:40
Mustafa SaidMustafa Said
3,0011913
3,0011913
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
add a comment |
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
$begingroup$
This is my solution for 2nd one but the answer is incorrect. $$ int{frac{dz}{z^2+9}} = int{frac{dz}{(z+3i)(z-3i)}} = int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{2pi i }} int {frac{{frac{dz}{z+3i}}}{z-3i}} = 2pi i {frac{1}{z+3i}} prime (at 3i) = {frac{-2pi i times (-1)}{(z+3i)^2}} = {frac{-2pi i}{(3i+3i)^2}} = {frac{-2pi i}{9+18i-9}} = {frac{-2pi}{-18i}} = {frac{pi}{9}} $$ The answer is ${frac{pi}{3}}$
$endgroup$
– user3132457
Dec 18 '18 at 4:48
add a comment |
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$begingroup$
The poles are at $pm 3i$. In each case, determine which poles are within the circle.
$endgroup$
– Lord Shark the Unknown
Dec 18 '18 at 4:36
$begingroup$
In both cases $3i$? If I draw that's what I get.
$endgroup$
– user3132457
Dec 18 '18 at 4:39