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Homework Statement
Evaluate the following integral,
I = \int_{0}^{2\pi} \frac{d \theta}{(1-2acos \theta + a^2)^2}, \ 0 < a < 1
For such, transform the integral above into a complex integral of the form ∫Rₐ(z)dz, where Rₐ(z) is a rational function of z. This will be obtained through the substitution z = e^iθ. Therefore, solve the complex integral through Cauchy's Integral Formula for the first derivative of an analytic function.
Homework Equations
Cauchy's Integral Formula
f^{(n)} (z_0) = \frac{n!}{2 \pi i} \oint_{\gamma} \frac{f(z)}{(z - z_0)^{(n+1)}}dz
The Attempt at a Solution
Writting dθ in terms of dz
z = e^{i \theta}
dz = ie^{i \theta} d\theta
\frac{1}{iz}dz = d\theta
Writting
\frac{1}{(1-2acos \theta + a^2)^2}
in terms of z, known that
\frac{1}{2}\left( z + \frac{1}{z}\right) = cos \theta
Therefore
\frac{1}{(1-a\left( z + \frac{1}{z}\right) + a^2)^2}
Thus
I = \oint_{\gamma} \frac{dz}{iz(1-a\left( z + \frac{1}{z}\right) + a^2)^2}
Where do I go from here? Any clues on how to express the denominator in terms of (z-z0)² ?
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