If the area A of the region bounded by the curves f ( x ) = 5 4 ( ∣ x ∣ − 1 − x 2 ) , ∣ x ∣ = 6 4 2 5 y 2 , and g ( x ) = 5 4 ( ∣ x ∣ − 3 1 − x 2 ) can be expressed as A = b a ( arcsin ( ϕ − 1 ) − c 1 ( ϕ − 1 ) d c ) , where a , b , c and d are coprime positive integers and ϕ is the golden ratio, find a + b + c + d .
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Using the symmetry about the y − a x i s we only need the right side ( x ≥ 0 ) of the graph.
For x ≥ 0 ⟹ f ( x ) = 5 4 ( x − 1 − x 2 ) , g ( x ) = 5 4 ( x − 3 1 − x 2 ) , and x = 6 4 2 5 y 2 ⟹ y = ± 5 8 x .
Let h ( x ) = 5 8 x and j ( x ) = 5 − 8 x .
I 1 = ∫ 0 2 5 − 1 ( f ( x ) − h ( x ) ) d x = 5 4 ∫ 0 2 5 − 1 ( 1 − x 2 − x ) d x
Let x = sin ( θ ) ⟹ d x = cos ( θ )
⟹ I 1 = 5 4 ( 2 1 arcsin ( 2 5 − 1 ) + 2 1 ( 2 5 − 1 ) 2 3 − 3 2 ( 2 5 − 1 ) 2 3 ) = 5 2 ( arcsin ( 2 5 − 1 ) − 3 1 ( 2 5 − 1 ) 2 3 ) .
I 2 = ∫ 0 2 5 − 1 ( j ( x ) − g ( x ) ) d x = 5 1 2 ∫ 0 2 5 − 1 ( 1 − x 2 − x ) d x = 3 I 1 ⟹
The desired area A = 2 ( 4 I 1 ) = 8 I 1 = 5 1 6 ( arcsin ( 2 5 − 1 ) − 3 1 ( 2 5 − 1 ) 2 3 ) .
Letting ϕ = 2 5 + 1 ⟹ A = 5 1 6 ( arcsin ( ϕ − 1 ) − 3 1 ( ϕ − 1 ) 2 3 ) = b a ( arcsin ( ϕ − 1 ) − c 1 ( ϕ − 1 ) d c ) ⟹ a + b + c + d = 2 6