Integral of the nth root of csc(x)

Calculus Level 3

0 π 2 csc θ n d θ = α n 2 β 1 + 1 n Γ ( 1 + γ n ) ( Γ ( 1 ϵ n ) ) δ \int_{0}^{\frac {\pi}{2}} \sqrt [n] {\csc \theta} d\theta = \frac {\alpha n^{2} \beta^{1 + \frac {1}{n}} \Gamma(1 + \frac {\gamma}{n})}{\left(\Gamma(\frac {1}{\epsilon n})\right)^{\delta}}

For ( n > 1 ) (n > 1) .

The above equality holds for real numbers α , β , γ , δ \alpha ,\beta, \gamma, \delta and ϵ \epsilon . Find ( 3 2 α β γ δ ϵ ) (-\frac {3}{2}\alpha\beta\gamma\delta - \epsilon) .

6.789 6 π + 2 6\pi+ 2 11 π + 7 11\pi + 7 -3.786 3 e 3e

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1 solution

Mark Hennings
Sep 19, 2020

We have (the integral only exists for n > 1 n>1 ) 0 1 2 π csc 1 n θ d θ = 1 2 B ( 1 2 1 2 n , 1 2 ) = Γ ( 1 2 1 2 n ) π 2 Γ ( 1 1 2 n ) = π n Γ ( 1 2 1 2 n ) Γ ( 1 2 n ) \int_0^{\frac12\pi}\csc^{\frac1n}\theta\,d\theta \; = \; \tfrac12B(\tfrac12 - \tfrac{1}{2n},\tfrac12) \; = \; \frac{\Gamma(\frac12-\frac{1}{2n})\sqrt{\pi}}{2\Gamma(1 - \frac{1}{2n})} \; = \; -\frac{\sqrt{\pi} n \Gamma(\frac12 - \frac{1}{2n})}{\Gamma(-\frac{1}{2n})} and since Γ ( 1 n ) = 1 2 π 2 1 2 1 n Γ ( 1 2 n ) Γ ( 1 2 1 2 n ) \Gamma(-\tfrac1n) = \tfrac{1}{\sqrt{2\pi}}2^{-\frac12-\frac1n}\Gamma(-\tfrac{1}{2n})\Gamma(\tfrac12 - \tfrac{1}{2n}) we deduce that the integral is equal to n π 2 1 + 1 n Γ ( 1 n ) Γ ( 1 2 n ) 2 = n 2 π 2 1 + 1 n Γ ( 1 1 n ) Γ ( 1 2 n ) 2 - \frac{n\pi 2^{1 + \frac1n}\Gamma(-\frac1n)}{\Gamma(-\frac{1}{2n})^2} \; = \; \frac{n^2\pi 2^{1+\frac1n}\Gamma(1-\frac1n)}{\Gamma(-\frac{1}{2n})^2} Thus α = π \alpha=\pi , β = 2 \beta=2 , γ = 1 \gamma=-1 , δ = 2 \delta=2 , ϵ = 2 \epsilon=-2 , giving the answer 6 π + 2 \boxed{6\pi+2} .

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