Wednesday, 14 May 2014

calculus - Compute intpi/20fraccosx2sin2xdx



How can I evaluate the following integral?




I=π/20cosx2sin2xdx








I tried it with Wolfram Alpha, it gave me a numerical solution: 0.785398.
Although I immediately know that it is equal to π/4, I fail to obtain the answer with pen and paper.
I tried to use substitution u=tanx, but I failed because the upper limit of the integral is π/2 and tanπ/2 is undefined.
So how are we going to evaluate this integral? Thanks.


Answer



Hint:



Knowing that sin2x=2sinxcosx and sin2x+cos2x=1. The integral can be expressed as



I=π/20cosx1+(sinxcosx)2 dx




then use substitution xπ2x, we have



I=π/20sinx1+(sinxcosx)2 dx



Add the two I's and let u=sinxcosx.


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