That's plausible

Calculus Level 4

0 sin x x d x = π A B C \large \int_0^\infty \dfrac{\sin x }{ \sqrt x} \, dx = \dfrac{\pi^A}{B^C}

If the equation above holds true for rational numbers A A and C C , with positive integer B B minimized, submit your answer as 1 A B C \dfrac1{ABC} .


The answer is 2.

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1 solution

Viki Zeta
Sep 30, 2016

Relevant wiki: Gamma Function

I = 0 sin ( x ) x d x Let, u = x d u d x = d d x x = 1 2 x d x = 2 x d u = 2 u d u I = 0 sin ( u 2 ) u 2 u d u I = 2 0 sin ( u 2 ) d u Now, use the property : 0 sin ( x a ) d x = Γ ( 1 + 1 a ) sin ( π 2 a ) [Fresnel_integral] I = 2 [ Γ ( 1 + 1 2 ) sin ( π 4 ) ] I = 2 [ Γ ( 3 2 ) 1 2 ] I = 2 [ π 2 1 2 ] I = π 2 π a b c = π 1 2 2 1 2 1 a b c = 1 1 2 . 1 2 . 2 = 2 ____________________________________________________________________ Γ ( n 2 ) = ( n 2 ) ! ! π 2 n 1 2 Γ ( 3 2 ) = ( 3 2 ) ! ! π 2 3 1 2 = π 2 2 2 = π 2 Γ ( n 2 ) = π 2 \displaystyle I = \int_0^{\infty} \dfrac{\sin(x)}{\sqrt{x}} ~~ dx \\ \text{Let, } u = \sqrt[]{x} \\ \dfrac{du}{dx} = \dfrac{d}{dx} \sqrt[]{x} = \dfrac{1}{2\sqrt[]{x}} \\ dx = 2\sqrt[]{x} ~ du = 2u du \\ I = \int_0^{\infty} ~ \dfrac{\sin(u^2)}{u} ~ 2u ~ du \\ I = 2~\int_0^{\infty}~\sin(u^2)~du \\ \text{Now, use the property : } \int_0^{\infty} ~ \sin(x^a) ~ dx ~= ~ \Gamma \left(1 + \dfrac{1}{a}\right)~\cdot ~ \sin\left(\dfrac{\pi}{2a}\right) \text{ [Fresnel\_integral]} \\ I = 2 \left[ \Gamma\left(1 + \dfrac{1}{2}\right)~\sin\left(\dfrac{\pi}{4}\right) \right] \\ I = 2 \left[ \Gamma\left(\dfrac{3}{2}\right) ~ \dfrac{1}{\sqrt[]{2}}\right] \\ I = 2 \left[ \dfrac{\sqrt[]{\pi}}{2} ~ \dfrac{1}{\sqrt[]{2}} \right] \\ I = \sqrt[]{\dfrac{\pi}{2}} \\ \therefore \dfrac{\pi^a}{b^c} = \dfrac{\pi^{\frac{1}{2}}}{2^{\frac{1}{2}}} \\ \boxed{\therefore \dfrac{1}{abc} = \dfrac{1}{\dfrac{1}{2} . \dfrac{1}{2} . 2} = 2}\\ \text{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_} \\ \Gamma\left(\dfrac{n}{2}\right) = \dfrac{(n-2)!! ~ \sqrt[]{\pi}}{2^{\frac{n-1}{2}}} \\ \Gamma\left(\dfrac{3}{2}\right) = \dfrac{(3-2)!! ~ \sqrt[]{\pi}}{2^{\frac{3-1}{2}}} \\ = \dfrac{\sqrt[]{\pi}}{2^{\frac{2}{2}}} = \dfrac{\sqrt[]{\pi}}{2} \\ \boxed{\therefore \Gamma\left(\dfrac{n}{2}\right) = \dfrac{\sqrt[]{\pi}}{2}}

Wow! This is a great solution!!!

Pi Han Goh - 4 years, 8 months ago

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Thanks. Hope this solution helps.

Viki Zeta - 4 years, 8 months ago

Can you please prove the identity you used for integrating S ( x ) S(x) ?

N. Aadhaar Murty - 9 months ago

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The identity is called Fresnel integral . There is a Brilliant Thread dedicated to proving this integral, https://brilliant.org/discussions/thread/proof-of-fresnel-integrals/

Viki Zeta - 8 months, 3 weeks ago

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