Integral of sin and cos invers

Calculus Level 5

0 1 4 arccos x arcsin x arccos x + arcsin x d x = a b c d π \large \int_{0}^{ \frac{1}{4}} \, \frac{\arccos{\sqrt{x}}-\arcsin{\sqrt{x}}}{\arccos{\sqrt{x}}+\arcsin{\sqrt{x}}}\, dx = \frac{a}{b}-\frac{\sqrt{c}}{d\pi}

The equation above holds true for coprime pairs of positive integers ( a , b ) , ( c , d ) (a,b), (c,d) . Find a + b + c + d a+b+c+d .


The answer is 22.

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2 solutions

Guilherme Niedu
Apr 19, 2017

I = 0 1 4 arccos ( x ) arcsin ( x ) arccos ( x ) + arcsin ( x ) d x \large \displaystyle I = \int_0^{\frac14} \frac{ \arccos(\sqrt{x}) - \arcsin(\sqrt{x}) } { \arccos(\sqrt{x}) + \arcsin(\sqrt{x}) } dx

u = arcsin ( x ) x = sin 2 ( u ) π 2 u = arccos ( x ) \color{#20A900} \boxed{\large \displaystyle u = \arcsin(\sqrt{x})} \rightarrow \boxed{\large \displaystyle x = \sin^2(u)} \rightarrow \boxed{\large \displaystyle \frac{\pi}{2} - u = \arccos(\sqrt{x})}

d u = 1 1 x 1 2 x d x d x = 2 1 sin 2 ( u ) sin 2 ( u ) d u d x = sin ( 2 u ) d u \color{#20A900} \large \displaystyle du = \frac{1}{\sqrt{1 - x}} \frac{1}{2\sqrt{x}} dx \rightarrow dx = 2 \sqrt{1 - \sin^2(u)}\sqrt{\sin^2(u)} du \rightarrow \boxed{\large \displaystyle dx = \sin(2u)du }

I = 0 π 6 π 2 2 u π 2 sin ( 2 u ) d u \large \displaystyle I = \int_0^{\frac{\pi}{6}} \frac{ \frac{\pi}{2} - 2u}{ \frac{\pi}{2}} \sin(2u) du

I = 0 π 6 sin ( 2 u ) d u 4 π 0 π 6 u sin ( 2 u ) d u \large \displaystyle I = \int_0^{\frac{\pi}{6}} \sin(2u) du - \frac{4}{\pi} \int_0^{\frac{\pi}{6}} u \sin(2u) du

I = cos ( 2 u ) 2 0 π 6 4 π ( u cos ( 2 u ) 2 0 π 6 + 0 π 6 cos ( 2 u ) 2 d u ) \large \displaystyle I = -\frac{\cos(2u)}{2} \Bigg |_0^{\frac{\pi}{6}} - \frac{4}{\pi} \left ( - \frac{u \cos(2u)}{2} \Bigg |_0^{\frac{\pi}{6}} + \int_0^{\frac{\pi}{6}} \frac{\cos(2u)}{2}du \right )

I = 1 4 4 π ( π 24 + sin ( 2 u ) 4 0 π 6 ) \large \displaystyle I = \frac14 - \frac{4}{\pi} \left ( \frac{\pi}{24} + \frac{\sin(2u)}{4} \Bigg |_0^{\frac{\pi}{6}} \right )

I = 1 4 4 π ( π 24 + 3 8 ) \large \displaystyle I = \frac14 - \frac{4}{\pi} \left ( \frac{\pi}{24} + \frac{\sqrt{3}}{8} \right )

I = 1 4 + 1 6 3 2 π \large \displaystyle I = \frac14 + \frac16 - \frac{\sqrt{3}}{2 \pi}

I = 5 12 3 2 π \color{#20A900} \boxed{ \large \displaystyle I = \frac{5}{12} - \frac{\sqrt{3}}{2 \pi} }

a = 5 , b = 12 , c = 3 , d = 2 , a + b + c + d = 22 \color{#3D99F6} \large \displaystyle a = 5, b = 12, c = 3, d = 2, \boxed { \large \displaystyle a + b + c + d = 22}

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