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=limn+πnn1k=1logsinπkn=πlog2.

There is also a well-known proof through symmetry:
I=π0log(sinx)=2π/20log(sin(2t))dt=2π/20log(2sintcost)dt=πlog2+2π/20log(sint)dt+2π/20log(cost)dt=πlog2+2I

from which I=πlog2 immediately follows.


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