Monday, 22 April 2013

calculus - Evaluating intpi0ln(1+cosx),dx





The problem is



π0ln(1+cosx) dx



What I tried was using standard limit formulas like changing x to πx and I also tried integration by parts on it to no avail. Please help. Also this is my first question so please tell if I am wrong somewhere.


Answer



π0log(1+cosx)dx=π/20log(1+cosx)dx+π/20log(1+cos(πx))dx=π/20log(sin2x)dx=π0log(sinx)dx
And by a notorious identity:
n1k=1sinkπn=2n2n,
hence the RHS of (1) can be computed as a Riemann sum:

π0log(sinx)dx=lim
There is also a well-known proof through symmetry:
\begin{eqnarray*}I=\int_{0}^{\pi}\log(\sin x)&=&2\int_{0}^{\pi/2}\log(\sin(2t))\,dt=2\int_{0}^{\pi/2}\log(2\sin t\cos t)\,dt\\&=&\pi\log 2+2\int_{0}^{\pi/2}\log(\sin t)\,dt+2\int_{0}^{\pi/2}\log(\cos t)\,dt\\&=&\pi \log 2 + 2I\tag{4}\end{eqnarray*}
from which I=-\pi\log 2 immediately follows.


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