Its too hard to come up with cool names for integrals

Calculus Level 5

Evaluate the following:

0 x 3 sin 3 x x 5 d x \int_0^{\infty} \frac{x^3-\sin^3x}{x^5}\,dx

If the result can be expressed as a π b c \dfrac{a\pi^b}{c} where a a and c c are coprime. Find a + b + c a+b+c .


The answer is 46.

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1 solution

Haroun Meghaichi
Jul 11, 2014

For a 0 a\geq 0 define : f ( a ) = 0 ( a x ) 3 sin 3 ( a x ) x 5 d x f(a)= \int_0^{\infty} \frac{(ax)^3-\sin^3 (ax)}{x^5} \ \mathrm{d}x Differentiate it 4 times (note that the third derivative is absolutely convergent, then we can differentiate) to get : f ( 4 ) ( a ) = 0 81 sin ( 3 a x ) 4 x 3 sin ( a x ) 4 x d x = π 2 ( 81 4 3 4 ) = 39 π 4 . f^{(4)}(a)= \int_0^{\infty} \frac{81\sin(3 ax)}{4x} -\frac{3\sin (ax)}{4x} \ \mathrm{d}x = \frac{\pi}{2}\left(\frac{81}{4}-\frac{3}{4} \right)= \frac{39\pi}{4}. Note that

f ( 0 ) = f ( 0 ) = f ( 2 ) ( 0 ) = f ( 3 ) ( 0 ) = 0 f(0)=f'(0)=f^{(2)}(0)=f^{(3)}(0)= 0 then : f ( a ) = 13 a 4 π 32 f(a) = \frac{13a^4\pi }{32}

Easily we get f ( 1 ) = 13 π 32 f(1)= \frac{13\pi}{32} , then answer is 13 + 1 + 32 = 46 13+1+32=\boxed{46} .

Another method : we have I = 1 4 0 4 x 3 + sin ( 3 x ) 3 sin x x 5 d x I=\frac{1}{4} \int_0^{\infty} \frac{4x^3 +\sin(3x)-3\sin x}{x^5} \ \mathrm{d}x Integrate by parts to get : 4 I = 0 4 x 3 + sin ( 3 x ) 3 sin x x 5 d x = 1 4 [ 4 x 3 + sin ( 3 x ) 3 sin x x 4 ] 0 = 0 + 1 4 0 12 x 2 3 cos x + 3 cos ( 3 x ) x 4 d x 4I= \int_0^{\infty} \frac{4x^3 +\sin(3x)-3\sin x}{x^5} \ \mathrm{d}x =\frac{-1}{4} \underbrace{\left[\frac{4x^3+\sin(3x)- 3 \sin x}{x^4} \right]_0^{\infty}}_{=0} +\frac{1}{4} \int_0^{\infty} \frac{12x^2 -3\cos x +3\cos(3x)}{x^4} \ \mathrm{d}x The brackets gives zero since near zero we have 4 x 3 + sin ( 3 x ) 3 sin x = 2 x 5 + o ( x 5 ) 4x^3+\sin (3x)-3\sin x=2x^5+o(x^5) by Taylor series, and the limit at \infty is obviously 0 0 , we repeat the same thing 16 I = 0 12 x 2 3 cos x + 3 cos ( 3 x ) x 4 d x = 1 3 0 24 x + 3 sin x 9 sin 3 x x 3 d x 16 I= \int_0^{\infty} \frac{12x^2 -3\cos x +3\cos(3x)}{x^4} \ \mathrm{d}x =\frac{1}{3} \int_0^{\infty} \frac{ 24x+3\sin x-9\sin 3x}{x^3} \ \mathrm{d}x And again two times : 48 I = 1 2 0 24 + 3 cos x 27 cos ( 3 x ) x 2 d x = 1 2 0 81 sin ( 3 x ) x 3 sin x x d x 48I = \frac{1}{2} \int_0^{\infty}\frac{24+3\cos x -27\cos(3x)}{x^2} \ \mathrm{d}x = \frac{1}{2} \int_0^{\infty} \frac{81\sin(3x)}{x} - \frac{3\sin x}{x} \ \mathrm{d}x We know that for any a 0 a\neq 0 0 sin a x x d x = π 2 \int_0^{\infty} \frac{\sin a x}{x} \ \mathrm{d}x = \frac{\pi}{2} (For a proof Google for Dirichlet integral), then : 96 I = ( 81 3 ) π 2 I = 13 π 32 . 96 I = \left(81-3\right)\frac{\pi}{2} \Longrightarrow I=\boxed{\frac{13\pi}{32}} .

Pff..i did what you said in the hint..that was long indeed..

Pratik Shastri - 6 years, 11 months ago

can you please give the solution by your second method

Aman Kumar - 6 years, 11 months ago

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@Aman Kumar I've added the second solution.

Haroun Meghaichi - 6 years, 11 months ago

I don't get this part: "Note that f(0)=f'(0)=... then: f(a)=..." Then what's 39pi/4?

John M. - 6 years, 11 months ago

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f ( 4 ) ( 0 ) f^{(4)}(0) isn't needed, we have f ( 3 ) ( a ) = 13 π 32 a + C f^{(3)}(a)= \frac{13\pi}{32}a + C , set a = 0 a=0 to get C = 0 C=0 and repeat this process.

Haroun Meghaichi - 6 years, 11 months ago

Did using the first method.Nice solution!

Akshay Bodhare - 6 years, 4 months ago

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