Inspired by Surya Prakash

Calculus Level 5

0 e 3 x 2 3 sin ( x 2 3 ) d x \large \displaystyle \int _{ 0 }^{ \infty }{ { e }^{ -\sqrt { 3 } { x }^{ \frac { 2 }{ 3 } } }\sin\left( { x }^{ \frac { 2 }{ 3 } } \right) \, dx }

Given that the integral above is equal to 3 π A B \dfrac {3\pi^A} B for rational numbers A A and B B , find the value of A × B A \times B .


The answer is 8.

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1 solution

Mark Hennings
Dec 17, 2015

Since 0 x μ 1 e β x sin δ x d x = Γ ( μ ) ( β 2 + δ 2 ) 1 2 μ sin ( μ tan 1 δ β ) \int_0^\infty x^{\mu-1} e^{-\beta x} \sin \delta x\,dx \; = \; \frac{\Gamma(\mu)}{(\beta^2 + \delta^2)^{\frac12\mu}} \sin\left(\mu \tan^{-1}\tfrac{\delta}{\beta}\right) for μ > 1 \mu > -1 and β > δ \beta > |\delta| , we see that (putting x = y 3 2 x = y^{\frac32} ), 0 e 3 x 2 3 sin ( x 2 3 ) d x = 3 2 0 y 1 2 e 3 y sin y d y = 3 2 Γ ( 3 2 ) ( 3 2 + 1 2 ) 3 4 sin ( 3 2 tan 1 1 3 ) = 3 2 1 2 π 2 3 2 sin 1 4 π = 3 16 π \begin{array}{rcl} \displaystyle\int_0^\infty e^{-\sqrt{3}x^{\frac23}} \sin\big(x^{\frac23}\big)\,dx& = & \displaystyle\tfrac32 \int_0^\infty y^{\frac12}\,e^{-\sqrt{3}y} \sin y \,dy \\ & = & \displaystyle\tfrac32 \frac{\Gamma(\frac32)}{(\sqrt{3}^2 + 1^2)^{\frac34}} \sin\left(\tfrac32\tan^{-1}\tfrac{1}{\sqrt{3}}\right) \\ & = & \displaystyle \tfrac32 \frac{\frac12\sqrt{\pi}}{2^{\frac32}} \sin\tfrac14\pi \\ & = & \tfrac{3}{16}\sqrt{\pi} \end{array}

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Brilliant Mathematics Staff - 5 years, 6 months ago

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Sir don't you think we are asked 1 2 1 32 \frac{1}{2}*\frac{1}{32} , I believe it should be A × B A\times B in the question.

Department 8 - 5 years, 6 months ago

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Agreed. Someone has changed the question!

Mark Hennings - 5 years, 5 months ago

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@Mark Hennings Thanks for bringing this up. I checked that it's one of the moderators that made this error. Sorry for the inconvenience. I've made the relevant edits.

Brilliant Mathematics Staff - 5 years, 5 months ago

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