# Contour Integrals

• May 20th 2010, 03:33 AM
granma
Contour Integrals
I'm working on a problem that has contour integrals and checked the answers my professor provided with a solution sheet, however there's a part in the solution sheet I don't understand.

This is the question given:
http://img29.imageshack.us/img29/9885/problem1o.png

This is the solution:

http://img687.imageshack.us/img687/5355/solutionk.png

And this is the part I don't understand from this solution:

http://img97.imageshack.us/img97/903/confuzzle.png
Thanks in advance for any help.
• May 20th 2010, 03:47 AM
HallsofIvy
Quote:

Originally Posted by granma
I'm working on a problem that has contour integrals and checked the answers my professor provided with a solution sheet, however there's a part in the solution sheet I don't understand.

This is the question given:
http://img29.imageshack.us/img29/9885/problem1o.png

This is the solution:

http://img687.imageshack.us/img687/5355/solutionk.png

And this is the part I don't understand from this solution:

http://img97.imageshack.us/img97/903/confuzzle.png
Thanks in advance for any help.

Do you think possibly it was the "i" that was already in the integral: $\displaystyle (cos^2 \theta)(\underline{i} e^{i\theta})$?
• May 20th 2010, 04:34 AM
granma
Quote:

Originally Posted by HallsofIvy
Do you think possibly it was the "i" that was already in the integral: $\displaystyle (cos^2 \theta)(\underline{i} e^{i\theta})$?

Yes, that's what I think it is, and if that's the case there shouldn't be i^2 later on etc... I'm assuming it's probably just a typo? I think it's best if I directly contact my professor about this.
• May 20th 2010, 11:10 AM
AllanCuz
Quote:

Originally Posted by HallsofIvy
Do you think possibly it was the "i" that was already in the integral: $\displaystyle (cos^2 \theta)(\underline{i} e^{i\theta})$?

I don't think this is the case (though it appears likely),

$\displaystyle \int ( cos^2 \theta ) ( i e^{ i \theta } ) d \theta$

We need to get to the above from

$\displaystyle i \int ( \frac{1}{2} [ e^{i \theta } + e^{-i \theta} ]^2 )( i e^{ i \theta} ) d\theta$

Let's look at

$\displaystyle \frac{1}{2} [ e^{i \theta } + e^{-i \theta} ]^2$

$\displaystyle ( \frac{1}{2} [ e^{i \theta } + e^{-i \theta} ] )^2 = ( \frac{1}{2} [ (cos \theta + i sin \theta ) + (cos \theta - i sin \theta) ] )^2$

$\displaystyle ( \frac{1}{2} ( 2 cos \theta ) )^2 = cos^2 \theta$

Which transforms

$\displaystyle i \int ( \frac{1}{2} [ e^{i \theta } + e^{-i \theta} ]^2 )( i e^{ i \theta} ) d\theta = i \int cos^2 \theta (i e^{i \theta} ) d \theta$

We have an extra i here....I don't know where it came from but I'm relatively sure it didn't come from what was already in the integral.
• May 21st 2010, 04:10 AM
HallsofIvy
Quote:

Originally Posted by granma
Yes, that's what I think it is, and if that's the case there shouldn't be i^2 later on etc... I'm assuming it's probably just a typo? I think it's best if I directly contact my professor about this.

Oh, I see your point- the i also remained inside the integral! Yes, it most likely a typo- they intended to take the i outside the integral but accidently also left it inside.