Complicated integral... or is it?

Calculus Level 3

Evaluate:

π e 2 ln ( π ) sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) d x \large \int_{- \pi}^{e^{2 \ln(\sqrt{\pi})}} \frac{\sin \left( (x^3 - \pi x) \ln(x^2+1) \right)}{e^{-x^4+\pi} \cos(3x)} dx


The answer is 0.

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2 solutions

Chew-Seong Cheong
Aug 17, 2018

I = π exp ( 2 ln π ) sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) d x Note that e 2 ln π = e ln π = π = π π sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) d x Numerator is odd while the denominor is even = 0 the integrand is odd. \begin{aligned} I & = \int_{-\pi}^{\color{#3D99F6}\exp (2\ln \sqrt \pi)} \frac {\sin \left((x^3-\pi x)\ln (x^2+1)\right)}{e^{-x^4+\pi}\cos (3x)} dx & \small \color{#3D99F6} \text{Note that } e^{2\ln \sqrt \pi} = e^{\ln \pi} = \pi \\ & = \int_{-\pi}^{\color{#3D99F6}\pi} \frac {\color{#D61F06}\sin \left((x^3-\pi x)\ln (x^2+1)\right)}{\color{#3D99F6}e^{-x^4+\pi}\cos (3x)} dx & \small \color{#D61F06} \text{Numerator is odd} \color{#3D99F6} \text{ while the denominor is even} \\ & = \color{#D61F06} \boxed 0 & \small \color{#D61F06} \text{the integrand is odd.} \end{aligned}

This was doing my head in , but then when i looked at it really carefully, I noticed that it is really just zero. It took me 45 minutes to look at it and find the answer lol

Krishna Karthik - 2 years, 9 months ago

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You can plot the integrand to be sure.

Chew-Seong Cheong - 2 years, 9 months ago

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Yeah I should’ve done that

Krishna Karthik - 2 years, 9 months ago
Denis Kartachov
Aug 16, 2018

The integrand is an odd function, i.e.

f ( x ) = sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) f(x) = \frac{\sin \big( (x^3 - \pi x) \ln(x^2+1) \big)}{e^{-x^4+\pi} \cos(3x)}

then

f ( x ) = sin ( ( ( x ) 3 π ( x ) ) ln ( ( x ) 2 + 1 ) ) e e ( x ) 4 + π cos ( 3 ( x ) ) = sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) = f ( x ) f(-x) = \frac{\sin \big( ((-x)^3 - \pi (-x)) \ln((-x)^2+1) \big)}{e^{-e(-x)^4+\pi} \cos(3(-x))} = - \frac{\sin \big( (x^3 - \pi x) \ln(x^2+1) \big)}{e^{-x^4+\pi} \cos(3x)} = - f(x)

The integral bounds are π - \pi to e 2 ln ( π ) = e l n ( π ) = π e^{2 \ln(\sqrt{\pi})} = e^{ln(\pi)} = \pi

so

π e 2 ln ( π ) sin ( ( x 3 π x ) ln ( x 2 + 1 ) ) e x 4 + π cos ( 3 x ) d x = 0 \int_{-\pi}^{e^{2\ln(\sqrt{\pi})}} \frac{\sin \big( (x^3 - \pi x) \ln(x^2+1) \big)}{e^{-x^4+\pi} \cos(3x)} dx =0

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