Desired Area

Calculus Level 4

If the area A A of the region bounded by the curves f ( x ) = 4 5 ( x 1 x 2 ) , x = 25 64 y 2 f(x) = \dfrac{4}{5}(\sqrt{|x|} - \sqrt{1 - x^2}), |x| = \dfrac{25}{64}y^2 , and g ( x ) = 4 5 ( x 3 1 x 2 ) g(x) = \dfrac{4}{5}(\sqrt{|x|} - 3\sqrt{1 - x^2}) can be expressed as A = a b ( arcsin ( ϕ 1 ) 1 c ( ϕ 1 ) c d ) A = \dfrac{a}{b}(\arcsin(\phi - 1) - \dfrac{1}{c}(\phi - 1)^{\dfrac{c}{d}}) , where a , b , c a,b,c and d d are coprime positive integers and ϕ \phi is the golden ratio, find a + b + c + d a + b + c + d .

Refer to previous problem


The answer is 26.

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1 solution

Rocco Dalto
Mar 13, 2018

Using the symmetry about the y a x i s y-axis we only need the right side ( x 0 ) (x \geq 0) of the graph.

For x 0 f ( x ) = 4 5 ( x 1 x 2 ) , g ( x ) = 4 5 ( x 3 1 x 2 ) x \geq 0 \implies f(x) = \dfrac{4}{5}(\sqrt{x} - \sqrt{1 - x^2}), g(x) = \dfrac{4}{5}(\sqrt{x} - 3\sqrt{1 - x^2}) , and x = 25 64 y 2 y = ± 8 5 x x = \dfrac{25}{64}y^2 \implies y = \pm\dfrac{8}{5}\sqrt{x} .

Let h ( x ) = 8 5 x h(x) = \dfrac{8}{5}\sqrt{x} and j ( x ) = 8 5 x j(x) = \dfrac{-8}{5}\sqrt{x} .

I 1 = 0 5 1 2 ( f ( x ) h ( x ) ) d x = 4 5 0 5 1 2 ( 1 x 2 x ) d x I_{1} = \int_{0}^{\dfrac{\sqrt{5} - 1}{2}} (f(x) - h(x)) dx = \dfrac{4}{5}\int_{0}^{\dfrac{\sqrt{5} - 1}{2}} (\sqrt{1 - x^2} - \sqrt{x}) dx

Let x = sin ( θ ) d x = cos ( θ ) x = \sin(\theta) \implies dx = \cos(\theta)

I 1 = 4 5 ( 1 2 arcsin ( 5 1 2 ) + 1 2 ( 5 1 2 ) 3 2 2 3 ( 5 1 2 ) 3 2 ) = \implies I_{1} = \dfrac{4}{5}(\dfrac{1}{2}\arcsin(\dfrac{\sqrt{5} - 1}{2}) + \dfrac{1}{2}(\dfrac{\sqrt{5} - 1}{2})^{\dfrac{3}{2}} - \dfrac{2}{3}(\dfrac{\sqrt{5} - 1}{2})^{\dfrac{3}{2}}) = 2 5 ( arcsin ( 5 1 2 ) 1 3 ( 5 1 2 ) 3 2 ) \dfrac{2}{5}(\arcsin(\dfrac{\sqrt{5} - 1}{2}) - \dfrac{1}{3}(\dfrac{\sqrt{5} - 1}{2})^{\dfrac{3}{2}}) .

I 2 = 0 5 1 2 ( j ( x ) g ( x ) ) d x = 12 5 0 5 1 2 ( 1 x 2 x ) d x = 3 I 1 I_{2} = \int_{0}^{\dfrac{\sqrt{5} - 1}{2}} (j(x) - g(x)) dx = \dfrac{12}{5}\int_{0}^{\dfrac{\sqrt{5} - 1}{2}} (\sqrt{1 - x^2} - \sqrt{x}) dx = 3I_{1} \implies

The desired area A = 2 ( 4 I 1 ) = 8 I 1 = 16 5 ( arcsin ( 5 1 2 ) 1 3 ( 5 1 2 ) 3 2 ) A = 2(4I_{1}) = 8I_{1} = \dfrac{16}{5}(\arcsin(\dfrac{\sqrt{5} - 1}{2}) - \dfrac{1}{3}(\dfrac{\sqrt{5} - 1}{2})^{\dfrac{3}{2}}) .

Letting ϕ = 5 + 1 2 A = 16 5 ( arcsin ( ϕ 1 ) 1 3 ( ϕ 1 ) 3 2 ) = a b ( arcsin ( ϕ 1 ) 1 c ( ϕ 1 ) c d ) a + b + c + d = 26 \phi = \dfrac{\sqrt{5} + 1}{2} \implies A = \dfrac{16}{5}(\arcsin(\phi - 1) - \dfrac{1}{3}(\phi - 1)^{\dfrac{3}{2}}) = \dfrac{a}{b}(\arcsin(\phi - 1) - \dfrac{1}{c}(\phi - 1)^{\dfrac{c}{d}}) \implies a + b +c+ d = \boxed{26}

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