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Calculus Level 4

0 π 1024 10 sin 64 x d x = a π b ( c ! ) \large \int_{0}^{\frac{\pi}{\sqrt[10]{1024}}} \sin^{\sqrt{64}}x \ dx = \frac {a\pi}{b(c!)}

The equation above holds true for positive integers a a , b b and c c , where a a is as large as possible and co-prime to b b and c 1 c\neq 1 . Find the minimum value of a + b + c a+b+c .


The answer is 141.

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1 solution

Naren Bhandari
Apr 1, 2018

The integration 0 π 1024 10 sin 64 x d x \small\int_{0}^{\frac{\pi}{\sqrt[10]{1024}}} \sin^{\sqrt{64}}x\,dx can be simplified to 0 π 2 sin 8 x d x \small\int_{0}^{\frac{\pi}{2}} \sin^8 x\,dx . Before to integrate let us generalize the integration of 0 π 2 sin n x d x \small\int_{0}^{\frac{\pi}{2}} \sin^n x\,dx for any integer. n 1 n\geq 1 .

0 π 2 sin n x d x = 0 π 2 sin n . sin n 1 x d x 0 π 2 sin n x d x = cosx . sin n 1 x d x 0 π 2 + ( n 1 ) 0 π 2 cos 2 x . sin n 2 x d x c c c c c Integration by parts 0 π 2 sin n x d x = 0 + ( n 1 ) 0 π 2 cos 2 x . sin n 2 x d x 0 π 2 sin n x d x = ( n 1 ) 0 π 2 sin n 2 x d x ( n 1 ) 0 π 2 sin n x d x 0 π 2 sin n x d x = n 1 n 0 π 2 sin n 2 x d x I = n 1 n I n 2 = n 2 n . n 3 n 1 I n 4 = n 1 n . n 3 n 2 . n 5 n 4 I n 6 = \small\int_{0}^{\frac{\pi}{2}} \sin^n x\,dx = \int_{0}^{\frac{\pi}{2}}\sin^{n}. \sin^{n-1} x\,dx \\\int_{0}^{\frac{\pi}{2}} \sin^n x\,dx = - \text{cosx} . \sin^{n-1}x\,dx|_{0}^{\frac{\pi}{2}} +(n-1)\int_{0}^{\frac{\pi}{2}} \cos^2x .\sin^{n-2}x\,dx \phantom{ccccc}{\color{#3D99F6}\text{Integration by parts}} \\ \int_{0}^{\frac{\pi}{2}} \sin^n x\,dx = 0 +(n-1)\int_{0}^{\frac{\pi}{2}} \cos^2x .\sin^{n-2}x\,dx \\\int_{0}^{\frac{\pi}{2}} \sin^n x\,dx =(n-1) \int_{0}^{\frac{\pi}{2}} \sin^{n-2}x\,dx- (n-1)\int_{0}^{\frac{\pi}{2}}\sin^n x\,dx \\ \int_{0}^{\frac{\pi}{2}} \sin^n x\,dx = \frac{n-1}{n}\int_{0}^{\frac{\pi}{2}} \sin^{n-2}x\,dx \\ \implies I = \frac{n-1}{n} I_{n-2} =\frac{n-2}{n}.\frac{n-3}{n-1} I_{n-4} = \frac{n-1}{n}.\frac{n-3}{n-2}.\frac{n-5}{n-4}I_{n-6} = \cdots Now integrating continuesly using Integration by parts till sin x \sin^{\circ} x if n n is an even number and sinx d x \text{sinx}\,dx if n n is odd number we obtained as 0 π 2 sin n d x = { n 1 n . n 3 n 2 . n 5 n 4 π 2 c c c c if n is even n 1 n . n 3 n 2 . n 5 n 4 1 c c c c if n > 1 is odd \int_{0}^{\frac{\pi}{2}} \sin^n\,dx = \begin{cases}\frac{n-1}{n}.\frac{n-3}{n-2}.\frac{n-5}{n-4} \cdots \frac{\pi}{2} \phantom{cccc}{\color{#3D99F6}\text{if n is even}} \\ \frac{n-1}{n}.\frac{n-3}{n-2}.\frac{n-5}{n-4} \cdots 1 \phantom{cccc}{\color{#3D99F6}\text{if n > 1 is odd }} \end{cases} Since in the case we find n = 8 n=8 an even number so 0 π 2 sin 8 x d x = 7.5.3.1 π 2.4.6.8 = 105 32 ( 4 ! ) \int_{0}^{\frac{\pi}{2}} \sin^8x\,dx = \frac{7.5.3.1\pi}{2.4.6.8} =\frac{105}{32(4!)} . Hence the value of a + b + c = 105 + 32 + 4 = 141 a+b+c = 105+32+4=\boxed{141} .

@Naren Bhandari Well yeah, but this is where Beta Function simplifies things...........Also, Typo in the last step, you missed the pi........

Aaghaz Mahajan - 3 years ago

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