# Integration using residue theorem

1. Jul 23, 2014

### MissP.25_5

Hi. I have to use the residue theorem to integrate f(z).
Can someone help me out? I am stuck on the factorization part.

Find the integral
$$\int_{0}^{2\pi} \,\frac{d\theta}{25-24\cos\left({\theta}\right)}$$

My answer:
$$\int_{0}^{2\pi} \,\frac{d\theta}{25-24\cos\left({\theta}\right)}=\oint_{c}^{} \,\frac{dz/iz}{25-24(\frac{1}{2}(z+\frac{1}{z}))}$$

$$=\frac{1}{i}\oint_{c}^{} \,\frac{dz}{-12z^2+25z-12}$$

2. Jul 23, 2014

### vela

Staff Emeritus
Try something like $-12z^2 + 25z -12 = -(12z^2-25z+12) = -(3z+\cdots)(4z+\cdots)$.

3. Jul 23, 2014

### MissP.25_5

Got it.

Here's my continuation:

$$\frac{1}{i}\oint_{c}^{}\frac{dz}{-(4z-3)(3z-4)}=-\frac{1}{i}\oint_{c}^{}\frac{dz}{(4z-3)(3z-4)}$$

It has a singularity at z=3/4

$$Resf(z)_{|z=\frac{3}{4}|}=\lim_{{z}\to{\frac{3}{4}}}(z-\frac{3}{4})(\frac{1}{(4z-3)(3z-4)})=\frac{-7}{5}$$

By residue theorem, the integral becomes
$$2\pi{i}\frac{-1}{i}\frac{-7}{5}=\frac{14\pi}{5}$$

Is this correct? Check please.

4. Jul 23, 2014

### MissP.25_5

I had a miscalculation. The final answer should be $2\pi/7$.

5. Jul 24, 2014

### vela

Staff Emeritus
Looks good.

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