Does an elementary method exist?

Calculus Level 5

sin x x 2 + 4 x + 5 d x \large{\displaystyle \int^{\infty}_{- \infty} \frac{\sin x}{x^2+4x+5}\, dx}

If the value of the above integral is equal to π A e B sin C - \pi^{A} e^{B} \sin C where A , B A,B and C C are integers, find A × B × C A \times B \times C .


The answer is -2.

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2 solutions

Mark Hennings
Apr 19, 2016

It depends on what you call elementary. Here is a solution without contour integration.

The function f ( x ) = e x f(x) \,=\, e^{-|x|} has Fourier transform ( F f ) ( t ) = 1 2 π R f ( x ) e i x t d x = 2 π 1 1 + t 2 (\mathcal{F}f)(t) \; = \; \frac{1}{\sqrt{2\pi}}\int_{\mathbb{R}} f(x)e^{-ixt}\,dx \; = \; \sqrt{\frac{2}{\pi}} \frac{1}{1+t^2} and so, since the integrand is even, R cos x x 2 + 1 d x = π 2 R ( F f ) ( x ) e i x d x = π f ( 1 ) = π e \int_{\mathbb{R}} \frac{\cos x}{x^2+1}\,dx \; = \; \sqrt{\frac{\pi}{2}}\int_{\mathbb{R}}(\mathcal{F}f)(x)e^{ix}\,dx \; = \; \pi f(1) \; = \; \frac{\pi}{e} Thus the integral we want is R sin x x 2 + 4 x + 5 d x = R sin ( x 2 ) x 2 + 1 d x = sin 2 R cos x x 2 + 1 d x = π e 1 sin 2 \int_{\mathbb{R}}\frac{\sin x}{x^2 + 4x + 5}\,dx \; = \; \int_{\mathbb{R}}\frac{\sin(x-2)}{x^2+1}\,dx \; = \; -\sin 2\int_{\mathbb{R}}\frac{\cos x}{x^2+1}\,dx \; = \; -\pi e^{-1} \sin 2 making the answer 1 × 1 × 2 = 2 1 \times -1 \times 2 = \boxed{-2} .

Pi Han Goh
Apr 22, 2016

sin x x 2 + 4 x + 5 d x = sin x ( x + 2 ) 2 + 1 d x = y = x + 2 sin ( y 2 ) y 2 + 1 d y = sin y cos 2 cos y sin 2 y 2 + 1 d y = cos 2 sin y y 2 + 2 d y = 0 ( odd function ) sin 2 cos y y 2 + 1 d y = sin 2 cos y y 2 + 1 d y \begin{aligned} \int_{-\infty}^\infty \dfrac{ \sin x}{x^2+4x+ 5} \, dx &=& \int_{-\infty}^\infty \dfrac{ \sin x}{(x+2)^2+1} \, dx \stackrel{y=x+2}{\LARGE =} \int_{-\infty}^\infty \dfrac{ \sin (y-2)}{y^2+1} \, dy \\ &=& \int_{-\infty}^{\infty} \dfrac{ \sin y \cos 2 - \cos y \sin 2}{y^2+1} \, dy \\ &=& \cos 2 \underbrace{ \int_{-\infty}^\infty \dfrac{\sin y}{y^2+2} \, dy }_{=0 \; (\because \text{ odd function})}- \sin 2 \int_{-\infty}^\infty \dfrac{\cos y}{y^2 + 1} \, dy \\ &=&- \sin 2 \int_{-\infty}^\infty \dfrac{\cos y}{y^2 + 1} \, dy \end{aligned}

Claim : cos y y 2 + 1 d y = π e \displaystyle \int_{-\infty}^\infty \dfrac{\cos y}{y^2 + 1} \, dy = \dfrac \pi e .

Proof : Consider the Laplace transform of the function g ( a ) = 0 cos ( a x ) 1 + x 2 d x \displaystyle g(a) = \int_0^\infty \dfrac{ \cos(ax) }{1+x^2} \, dx ,

L ( g ( a ) ) = 0 0 cos ( a x ) 1 + x 2 e a s d a d x = 0 s ( 1 + x 2 ) ( s 2 + x 2 ) d x = π 2 ( s + 1 ) . \mathcal L (g(a)) = \int_0^\infty \int_0^\infty \dfrac{ \cos(ax)}{1+x^2} e^{-as} \, da \; dx = \int_0^\infty \dfrac s{(1+x^2)(s^2+x^2) } \, dx = \dfrac \pi{2(s+1)} \; .

Then g ( a ) = L 1 ( π 2 ( s + 1 ) ) = π 2 e a cos ( x ) 1 + x 2 d x = 2 g ( 1 ) = π e \displaystyle g(a) = \mathcal L^{-1} \left( \dfrac \pi{2(s+1)} \right) =\dfrac \pi 2 e^{-|a|} \Rightarrow \int_{-\infty}^\infty \dfrac{ \cos(x) }{1+x^2} \, dx = 2g(1) = \dfrac \pi e .

Hence, sin x x 2 + 4 x + 5 d x = sin 2 π e \displaystyle \int_{-\infty}^\infty \dfrac{ \sin x}{x^2+4x+ 5} \, dx = - \sin 2 \cdot \dfrac \pi e .

Pretty cool!!

Tanishq Varshney - 5 years, 1 month ago

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Thanks. I was actually planning on doing the standard "differentiate through the integral" approach, but I thought I can spice things up!

Pi Han Goh - 5 years, 1 month ago

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