# Contour Integrals

#### 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: This is the solution: And this is the part I don't understand from this solution: Thanks in advance for any help.

#### HallsofIvy

MHF Helper
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: This is the solution: And this is the part I don't understand from this solution: Thanks in advance for any help.
Do you think possibly it was the "i" that was already in the integral: $$\displaystyle (cos^2 \theta)(\underline{i} e^{i\theta})$$?

• granma

#### granma

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.

#### AllanCuz

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.

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#### HallsofIvy

MHF Helper
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.