The problem is
∫π0ln(1+cosx) dx
What I tried was using standard limit formulas like changing x to π−x 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(π−x))dx=∫π/20log(sin2x)dx=∫π0log(sinx)dx
And by a notorious identity:
n−1∏k=1sinkπn=2n2n,
hence the RHS of (1) can be computed as a Riemann sum:
∫π0log(sinx)dx=limn→+∞πnn−1∑k=1logsinπkn=−πlog2.
There is also a well-known proof through symmetry:
I=∫π0log(sinx)=2∫π/20log(sin(2t))dt=2∫π/20log(2sintcost)dt=πlog2+2∫π/20log(sint)dt+2∫π/20log(cost)dt=πlog2+2I
from which I=−πlog2 immediately follows.
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