This one is not that bad

Calculus Level 4

2 2 1 1 x 5 4 x 2 1 d x \large \int_{\frac{\sqrt 2}2}^1 \dfrac1{x^5 \sqrt{4x^2-1}} \, dx

If the integral above can be expressed as A + B C A + π D -A + \dfrac {B\sqrt C}A + \dfrac \pi D , where A A , B B , C C and D D are positive integers with A A and B B being coprime integers, and C C square-free, find A + B + C + D A+B+C+D .

18 17 19 16

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1 solution

Chew-Seong Cheong
Jun 17, 2016

I = 1 2 1 1 x 5 4 x 2 1 d x Let sec u = 2 x sec u tan u d u = 2 d x = π 4 π 3 16 sec u tan u sec 5 u sec 2 u 1 d u sec 2 u 1 = tan 2 u = tan u = 16 π 4 π 3 cos 4 u d u = 16 π 4 π 3 ( cos 2 u + 1 2 ) 2 d u = 4 π 4 π 3 ( cos 2 2 u + 2 cos 2 u + 1 ) d u = 4 π 4 π 3 ( 1 2 ( cos 4 u + 1 ) + 2 cos 2 u + 1 ) d u = 2 π 4 π 3 ( cos 4 u + 4 cos 2 u + 3 ) d u = 2 ( sin 4 u 4 + 2 sin 2 u + 3 u ) π 4 π 3 = 1 2 ( 3 2 0 ) + 4 ( 3 2 1 ) + 6 ( π 3 π 4 ) = 4 + 7 3 4 + π 2 \begin{aligned} I & = \int_\frac 1{\sqrt 2}^1 \frac 1{x^5\sqrt{4x^2-1}} \ dx \quad \quad \small \color{#3D99F6}{\text{Let } \sec u = 2x \quad \sec u \tan u \ du = 2 \ dx} \\ & = \int_\frac \pi 4^\frac \pi 3 \frac {16 \sec u \tan u}{\sec^5 u \color{#3D99F6}{\sqrt{\sec^2 u -1}}} \ du \quad \quad \small \color{#3D99F6}{\sqrt{\sec^2 u -1} = \sqrt{\tan^2 u} = \tan u} \\ & = 16 \int_\frac \pi 4^\frac \pi 3 \cos^4 u \ du \\ & = 16 \int_\frac \pi 4^\frac \pi 3 \left(\frac{\cos 2u +1}2 \right)^2 \ du \\ & = 4 \int_\frac \pi 4^\frac \pi 3 \left(\cos^2 2u + 2\cos 2u + 1 \right) \ du \\ & = 4 \int_\frac \pi 4^\frac \pi 3 \left(\frac 12 \left( \cos 4u + 1 \right) +2\cos 2u + 1 \right) \ du \\ & = 2 \int_\frac \pi 4^\frac \pi 3 \left(\cos 4u +4\cos 2u + 3 \right) \ du \\ & = 2 \left(\frac{\sin 4u}4 +2\sin 2u + 3u \right) \bigg|_\frac \pi 4^\frac \pi 3 \\ & = \frac 12 \left(-\frac {\sqrt{3}}2 - 0 \right) + 4 \left(\frac {\sqrt{3}}2 -1 \right) + 6 \left(\frac \pi 3 -\frac \pi 4 \right) \\ & = -4 + \frac {7\sqrt 3}4 + \frac \pi 2 \end{aligned}

A + B + C + D = 4 + 7 + 3 + 2 = 16 \implies A+B+C+D = 4+7+3+2 = \boxed{16}

Very nice solution. I also solved it the same way.

Hana Wehbi - 4 years, 12 months ago

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Nice question, reshared.

Ashish Menon - 4 years, 12 months ago

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Thank you.

Hana Wehbi - 4 years, 12 months ago

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@Hana Wehbi You're welcome

Ashish Menon - 4 years, 12 months ago

Solved it the same way (+1)

Ashish Menon - 4 years, 12 months ago

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