Complete Elliptic Integral Of The First Kind?

Calculus Level 5

1 2 π / 2 π / 2 d θ 1 ( 2 1 ) 2 sin 2 θ = A + B A C / D π Γ ( B E ) Γ ( F E ) \dfrac{1}{2}\int_{-\pi/2}^{\pi/2}\dfrac{d\theta}{\sqrt{1-(\sqrt{2}-1)^2\sin^2{\theta}}}=\dfrac{\sqrt{\sqrt{A}+B}}{A^{C/D}\sqrt{\pi}}\Gamma\left(\dfrac{B}{E}\right)\Gamma\left(\dfrac{F}{E}\right)

The equation above holds true positive integers A , B , C , D , E A,B,C,D,E and F F such that A A is square-free and C , D C,D are coprime. Find A + B + C + D + E + F A+B+C+D+E+F .

Notation : Γ ( ) \Gamma(\cdot) denotes the Gamma function .


The answer is 31.

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1 solution

First apply the following substitution θ = arctan ( 1 2 ( x 1 / 8 x 1 / 8 2 1 / 4 2 1 / 4 x 1 / 8 x 1 / 8 ) ) \theta=\arctan{\left(\frac{1}{2}\left(\frac{x^{-1/8}-x^{1/8}}{2^{1/4}}-\frac{2^{1/4}}{x^{-1/8}-x^{1/8}}\right)\right)} It is easy to check that x runs from 0 to 1 and the integral becomes 2 + 1 2 13 / 4 0 1 x 7 / 8 + x 5 / 8 x + 1 d x = 2 + 1 2 13 / 4 0 x 7 / 8 x + 1 d x \frac{\sqrt{\sqrt{2}+1}}{2^{13/4}}\int_0^1\frac{x^{-7/8}+x^{-5/8}}{\sqrt{x+1}}dx=\frac{\sqrt{\sqrt{2}+1}}{2^{13/4}}\int_0^\infty\frac{x^{-7/8}}{\sqrt{x+1}}dx Then the claim follows by the following formula 0 x a 1 ( 1 + x ) b d x = Γ ( a ) Γ ( b a ) Γ ( b ) \int_0^\infty \frac{x^{a-1}}{(1+x)^b}dx=\frac{\Gamma(a)\Gamma(b-a)}{\Gamma(b)} then 1 2 π / 2 π / 2 d θ 1 ( 2 1 ) 2 sin 2 θ = 2 + 1 2 13 / 4 π Γ ( 1 8 ) Γ ( 3 8 ) \frac{1}{2}\int_{-\pi/2}^{\pi/2}\frac{d\theta}{\sqrt{1-(\sqrt{2}-1)^2\sin^2{\theta}}}=\frac{\sqrt{\sqrt{2}+1}}{2^{13/4}\sqrt{\pi}}\Gamma\left(\frac{1}{8}\right)\Gamma\left(\frac{3}{8}\right) and A + B + C + D + E + F = 2 + 1 + 13 + 4 + 8 + 3 = 31 A+B+C+D+E+F=2+1+13+4+8+3=\fbox{31}

How do you know about the substitution ???

Kushal Bose - 4 years, 11 months ago

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@Santiago Hincapie , Kushal is right. What motivates you to use that substitution?

Pi Han Goh - 4 years, 11 months ago

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