# Unit circle and the roots

## Homework Statement

Calculate the integral ∫dθ/(1+acos(θ)) from 0 to 2∏ using residues.

## Homework Equations

Res$\underline{zo}$(z)=lim$\underline{z->zo}$ (z-z0)f(zo)*2∏i

## The Attempt at a Solution

To start I sub cos(θ)=1/2(e^(iθ)+e^(-iθ)) so that de^(iθ)=ie^(iθ)dθ

Re-writing in terms of e^(iθ) and then subbing in z for e^(iθ) I get dz/(iz(1+a(1/2)(z+z*))

This becomes -2idz/(az^2+2z+a). Solving for the pole I get two imaginary roots. The problem is this is all done on the unit circle and the roots I'm getting depend on a so it means the poles are not necessarily in the circle. One should be. 0 is another root but is trivial.

Also I tried solving this integral numerically and it gave me 0 but the answer should be 2∏/√(1-a^2)

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haruspex
Homework Helper
Gold Member
2020 Award

I think the answer depends on whether a > 1. If it is, the roots are complex and both lie inside the unit circle, and they cancel. If < 1, only one root lies inside the circle, so you ignore the other.

vela
Staff Emeritus
Homework Helper

Solving for the pole I get two imaginary roots. The problem is this is all done on the unit circle and the roots I'm getting depend on a so it means the poles are not necessarily in the circle.
When |a|>1, you should get two complex roots, namely ##z=-\frac{1}{a} \pm i\sqrt{1-\left(\frac{1}{a}\right)^2}##. You should be able to convince yourself that both roots lie on the unit circle.

0 is another root but is trivial.
z=0 isn't a root.

Also I tried solving this integral numerically and it gave me 0 but the answer should be 2∏/√(1-a^2)
From the answer, you should see that you must have |a|<1, so that's the case you're primarily interested in.