Let and .
If and have common tangents at points and and and respectively, find the area of the region above to eight decimal places.
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Let f ( x ) = ∣ x ∣ and g ( x ) = − x 2 and A : ( x 0 , y 0 ) .
R ( 0 , 0 ) ∘ R y = x ( x 0 , y 0 ) = ( − y 0 , − x 0 ) = B
and R y − a x i s ( − y 0 , − x 0 ) = ( y 0 , − x 0 ) = D and R y − a x i s ( x 0 , y 0 ) = ( − x 0 , y 0 ) = C
⟹ m A B = 1 and m C D = − 1
m A B = 1 ⟹ f ′ ( x 0 ) = 2 x 0 1 = 1 ⟹ x 0 = 4 1 ⟹ y 0 = 2 1 ⟹ y = x + 4 1
and m C D = − 1 ⟹ y = − x + 4 1
To find the x coordinate of the find point of intersection E of y = − x + 4 1 and f ( x ) = x
let − x + 4 1 = x ⟹ 1 − 4 x = 4 x ⟹ 1 − 8 x + 1 6 x 2 = 1 6 x ⟹ 1 6 x 2 − 2 4 x + 1 = 0 ⟹ x = 4 3 ± 2 2
We drop x = 4 3 + 2 2 which doesn't satisfy the initial equation and choose x = 4 3 − 2 2 which satisfies the initial equation.
For A R 1 we have:
A R 1 = ∫ 0 4 1 x + 4 1 − x 2 1 d x = 2 1 x 2 + 4 1 x − 3 2 x 2 3 ∣ 0 4 1 = 9 6 1
For A R 2 we have:
A R 2 = ∫ 0 4 3 − 2 2 x 2 1 + x 2 d x = 3 2 x 2 3 + 3 1 x 3 ∣ 0 4 3 − 2 2 =
3 2 ( 4 3 − 2 2 ) 2 3 + 3 1 ( 4 3 − 2 2 ) 3 .
For A R 3 we have:
A R 3 = ∫ 4 3 − 2 2 2 1 ( − x + 4 1 + x 2 ) d x =
− 2 1 x 2 + 4 1 x + 3 1 x 3 ∣ 4 3 − 2 2 2 1 =
2 4 1 − ( − 2 1 ( 4 3 − 2 2 ) 2 + 4 1 ( 4 3 − 2 2 ) + 3 1 ( 4 3 − 2 2 ) 3 )
A R 2 + A R 3 = ( 4 3 − 2 2 ) ( 2 4 8 3 − 2 2 + 3 − 6 2 ) + 2 4 1
The Area A ∗ = A R 1 + A R 2 + A R 3 = 9 6 1 [ 5 + ( 3 − 2 2 ) ( 8 3 − 2 2 + 3 − 6 2 ) ]
Using the symmetry about the y axis the total area A = 2 A ∗ =
9 6 2 [ 5 + ( 3 − 2 2 ) ( 8 3 − 2 2 + 3 − 6 2 ) ] ≈ 0 . 0 9 6 4 0 4 5 2