Sunday, 29 September 2019

integration - Evaluating intfracpi2fracpi4(2csc(x))17dx



I saw this question in JEE Advanced. But in that we had to simplify it to log(1+2)02(eu+eu)16du


But I pose the following question as to evaluate this integral in closed form.
My Attempt:
π2π4(2csc(x))17dx

=π2π4(2.2i)17(eixeix)17dx

(Bit of a cheeky attempt! I know. :D)
417iπ2π4e17ixe2ix1dx


If we put u=eix;du=i.eixdx, then 417i1+i2u16u21du

(Huh??!!)
4171+i2iu16u21du

And u16u21=u1515+u1313+u1111+u99+u77+u55+u33+u+12log(1u)12log(u+1)+C

But at this point I am stuck. Please can someone help?


Answer



By applying the Weierstrass substitution x=2arctant we have:
I=π/2π/4(2sinx)17dx=π/2π/4(1sinx2cosx2)17=2121(1+t2t)17dt1+t2


then by setting t=eu and applying the binomial theorem it follows that:
I=2log(1+2)0(eu+eu)16du=1603731627+25740log(1+2).



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