A calculus problem by Akshat Sharda

Calculus Level 3

0 1 x tan 1 x ( 1 + x 2 ) 3 / 2 d x \int^{1}_{0} \dfrac{x\tan^{-1}x}{(1+x^2)^{3/2}} \mathrm{d} x

If the answer to the above problem is 1 n π 4 2 \dfrac{1}{\sqrt{n}}-\dfrac{\pi}{4\sqrt{2}} , then find the value of n n .


The answer is 2.

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1 solution

I = 0 1 x tan 1 x ( 1 + x 2 ) 3 2 d x Let x = tan θ d x = sec 2 θ d θ = 0 π 4 θ tan θ sec 2 θ ( 1 + tan 2 θ ) 3 2 d θ Note that sec 2 θ = 1 + tan 2 θ = 0 π 4 θ tan θ sec θ d θ Note that sec θ = 1 cos θ = 0 π 4 θ sin θ d θ By integration by parts = θ cos θ 0 π 4 + 0 π 4 cos θ d θ = π 4 1 2 + sin θ 0 π 4 = π 4 2 + 1 2 0 = 1 2 π 4 2 \begin{aligned} I & = \int_0^1 \frac {x\tan^{-1}x}{(1+x^2)^\frac 32} \ dx & \small \color{#3D99F6} \text{Let }x = \tan \theta \implies dx = \sec^2 \theta \ d \theta \\ & = \int_0^\frac \pi 4 \frac {\theta \tan \theta \sec^2 \theta }{(1+\tan^2 \theta)^\frac 32} \ d \theta & \small \color{#3D99F6} \text{Note that } \sec^2 \theta = 1+ \tan^2 \theta \\ & = \int_0^\frac \pi 4 \frac {\theta \tan \theta}{\sec \theta} \ d \theta & \small \color{#3D99F6} \text{Note that } \sec \theta = \frac 1{\cos \theta} \\ & = \int_0^\frac \pi 4 \theta \sin \theta \ d \theta & \small \color{#3D99F6} \text{By integration by parts} \\ & = - \theta \cos \theta \bigg|_0^\frac \pi 4 + \int_0^\frac \pi 4 \cos \theta \ d \theta \\ & = - \frac \pi 4 \cdot \frac 1{\sqrt 2} + \sin \theta \bigg|_0^\frac \pi 4 \\ & = - \frac {\pi}{4 \sqrt 2} + \frac 1{\sqrt 2} - 0 \\ & = \frac 1{\sqrt 2} - \frac {\pi}{4 \sqrt 2} \end{aligned}

n = 2 \implies n = \boxed{2}

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