This Year is 2017

Calculus Level 5

0 π 2 tan x 2017 d x \large \int^\frac \pi 2_0 \sqrt [2017]{\tan x}\ dx

If the closed form of the integral above can be represented as a π b csc ( π l m ) \dfrac{a\pi}{b} \csc \bigg(\dfrac{\pi l}{m} \bigg) , where a , b , m , l Z + a,b,m,l \in \mathbb{Z^{+}} with gcd ( a , b ) = 1 \gcd(a,b)=1 , gcd ( m , l ) = 1 \gcd(m,l)=1 and l l is odd integer also l < m l<m Find a + b + l + m a+b+l+m .

Bonus : Generalize for 0 π 2 tan x n d x \displaystyle \int^\frac \pi 2_0 \sqrt[n]{\tan x} \ dx .


The answer is 3029.

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1 solution

First Last
May 30, 2017

Using the Beta function:

0 π 2 sin 1 n ( x ) cos 1 n ( x ) d x = 1 2 β ( n + 1 2 n , n 1 2 n ) = Γ ( n + 1 2 n ) Γ ( n 1 2 n ) 2 Γ ( n + 1 + n 1 2 n ) \displaystyle\int_0^\frac{\pi}{2}\sin^\frac1{n}(x)\cos^\frac{-1}{n}(x)dx = \frac1{2}\beta(\frac{n+1}{2n},\frac{n-1}{2n}) = \frac{\Gamma(\frac{n+1}{2n})\Gamma(\frac{n-1}{2n})}{2\Gamma(\frac{n+1+n-1}{2n})}

Using Euler's Reflection Formula: Γ ( x ) Γ ( 1 x ) = π sin ( π x ) \displaystyle\Gamma(x)\Gamma(1-x)=\frac{\pi}{\sin(\pi x)}

0 π 2 sin 1 n ( x ) cos 1 n ( x ) d x = Γ ( 1 2 n + 1 2 ) Γ ( 1 2 1 2 n ) 2 = π 2 sin ( π ( 1 2 n + 1 2 ) ) \displaystyle\int_0^\frac{\pi}{2}\sin^\frac1{n}(x)\cos^\frac{-1}{n}(x)dx=\frac{\Gamma(\frac1{2n}+\frac1{2})\Gamma(\frac1{2}-\frac1{2n})}{2} = \frac{\pi}{2\sin(\pi( \frac1{2n}+\frac1{2}))}

Sub in n = 2017 for π 2 sin ( 1009 π 2017 ) \displaystyle\boxed{\frac{\pi}{2\sin(\frac{1009\pi}{2017})}}

Ya the best way is with Beta Function.

Md Zuhair - 4 years ago

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Yes Using beta functions is good way..But one can also use Contour integration to show that ( x m / ( 1 + x n ) d x = π / n c o s e c ( m + 1 ) π / n \int (x^m/(1+x^n)dx=π/n cosec(m+1)π/n limits from 0-infinity

Spandan Senapati - 4 years ago

Yes, beta makes it very easy.

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Ya. How is this question? I made it by myself. :)

Md Zuhair - 4 years ago

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Its great....keep posting !!! :)

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@A Former Brilliant Member Why your age is showing 14?

Md Zuhair - 4 years ago

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@Md Zuhair My age is 17

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@A Former Brilliant Member Oh i see...

Md Zuhair - 4 years ago

Is there a solution without beta function?

Samuel Shadrach - 3 years, 10 months ago

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Yes , I think you can use integration by parts.

A Former Brilliant Member - 3 years, 10 months ago

@A E Yes I tried but I couldnt do it. Can you give a rough sketches on how it's done?

Samuel Shadrach - 3 years, 10 months ago

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Let me try ....

A Former Brilliant Member - 3 years, 10 months ago

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