My thirteenth integration problem

Calculus Level 5

0 π x 3 sin ( π 4 x ) sin ( x ) sin ( π 4 + x ) cos ( x ) d x \large \displaystyle \int_{0}^{\pi} x^3 \sin \left( \dfrac{\pi}{4} -x \right) \sin (x) \sin \left( \dfrac{\pi}{4} + x \right) \cos (x) \, dx

If the above definite integral can be expressed in the form a π b c π d e , \dfrac{a \pi}{b} - \dfrac{c \pi^{d}}{e} , where a , b , c , d a,b,c,d and e e are all positive integers and gcd ( a , b ) = gcd ( c , e ) = 1 \gcd(a, b) = \gcd(c, e) = 1 , find a + b + c + d + e a+b+c+d+e .


The answer is 295.

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1 solution

Harsh Khatri
Feb 14, 2016

We simplify the given expression using trigonometric identities:

1 2 0 π x 3 ( 2 sin ( x ) cos ( x ) ) ( sin 2 ( π 4 ) cos 2 ( x ) cos 2 ( π 4 ) sin 2 ( x ) ) d x \displaystyle \Rightarrow \frac{1}{2} \displaystyle \int_{0}^{\pi} x^3 \Big( 2\sin(x)\cos(x) \Big) \Big( \sin^2(\frac{\pi}{4})\cos^2(x) - \cos^2(\frac{\pi}{4})\sin^2(x) \Big) dx

1 8 0 π x 3 ( 2 sin ( 2 x ) cos ( 2 x ) ) d x \displaystyle \Rightarrow \frac{1}{8} \displaystyle \int_{0}^{\pi} x^3 \Big( 2\sin(2x)\cos(2x) \Big) dx

1 8 0 π x 3 sin ( 4 x ) d x \displaystyle \Rightarrow \frac{1}{8} \displaystyle \int_{0}^{\pi} x^3 \sin(4x) dx

Applying Integration by parts repeatedly, we get:

( ( 3 x 256 x 3 32 ) cos ( 4 x ) + ( 3 x 2 128 3 1024 ) sin ( 4 x ) ) 0 π \displaystyle \Rightarrow \bigg( \Big( \frac{3x}{256} - \frac{x^3}{32} \Big) \cos(4x) + \Big( \frac{3x^2}{128} - \frac{3}{1024} \Big) \sin(4x) \bigg)_{0} ^{\pi}

3 π 256 π 3 32 \displaystyle \Rightarrow \frac{3\pi}{256} - \frac{\pi^3}{32}

a + b + c + d + e = 3 + 256 + 1 + 3 + 32 = 295 \displaystyle \Rightarrow a + b + c + d + e = 3 + 256 + 1 + 3 + 32 = \boxed{295}

Did exactly the same!!

Syed Shahabudeen - 5 years, 3 months ago

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