Thursday, 24 January 2013

calculus - How can one show that intpi/40sqrtsin(2x)overcos2(x)mathrmdx=2sqrt2overpicdotGamma2left(3over4right)?



Proposed:





π/40sin(2x)cos2(x)dx=22πΓ2(34)




My try:



Change (1) to



π/402sec2(x)tan(x)dx




π/402tan(x)+2tan3(x)dx



Not sure what substitution to use



How may we prove (1)?


Answer



By substituting x=arctant our integral takes the form:



I=102t1+t2dt



and by substituting 2t1+t2=u we get:
I=10(1+11u2)duu3/2

that is straightforward to compute through the substitution u2=s and Euler's Beta function:
I=12(4+πΓ(14)Γ(14)).

The identities Γ(z+1)=zΓ(z) and Γ(z)Γ(1z)=πsin(πz) settle OP's (1).


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