6
$\begingroup$

I have to verify that

$\int_0^\pi \ln(1+\alpha\cos(x))dx=\pi\ln\left(\frac{1+\sqrt{1-\alpha^2}}{2}\right)$

with $|\alpha|<1$. It is my homework and don't know where to begin.

  • 0
    @Ned Dabby: no problem. I'm glad to see such questions around.2012-09-11

2 Answers 2

4

$I(\alpha) = \int_0^{\pi} \ln (1+ \alpha \cos(x)) dx$ $\dfrac{dI}{d \alpha} = \int_0^{\pi} \dfrac{\cos(x)}{1+\alpha \cos(x)} dx = \dfrac1{\alpha} \int_0^{\pi} \dfrac{\alpha \cos(x)}{1+ \alpha \cos(x)} dx = \dfrac1{\alpha} \left( \pi - \int_0^{\pi} \dfrac{dx}{1+\alpha \cos(x)}\right)$ And integral $\int_0^{\pi} \dfrac{dx}{1+\alpha \cos(x)}$ can be evaluated using the standard complex analytic technique by using the transformation $z = \exp(ix)$.

  • 0
    @Thank you Marvis. Now, Iknow how to begin. I know that inyegral.2012-09-07
2

Hint: Differentiate the left-hand sign with respect to $\alpha$. The details are probably in your book, but if not, there is a useful Wikipedia article. You will get something you can integrate explicitly with respect to $x$.

Compare with the derivative of the right-hand side with respect to $\alpha$. Finally, observe that the left-hand side and the right-hand side agree at $\alpha=0$.