The pink parabola is tangent to a circle with radius at points and as shown above and the green parabola intersects the circle at points and .
Find the sum of the areas of regions and .
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Using the rotation equations:
x = x ′ cos ( θ ) − y ′ sin ( θ )
y = x ′ sin ( θ ) + y ′ cos ( θ )
For x 2 + 2 3 x y + 3 y 2 + 2 3 x − 2 y = 0 we have:
x ′ cos 2 ( θ ) − x ′ y ′ sin ( 2 θ ) + y ′ sin 2 ( θ ) + 3 ( x ′ 2 sin 2 ( θ ) + x ′ y ′ sin ( 2 θ ) + y ′ sin 2 ( θ ) ) +
2 3 ( x ′ y ′ cos ( 2 θ ) + ( x ′ 2 − y ′ 2 ) sin ( θ ) cos ( θ ) ) + 2 3 ( x ′ cos ( θ ) − y ′ sin ( θ ) )
− 2 ( x ′ sin ( θ ) + y ′ cos ( θ ) ) = 0
Setting x ′ y ′ term to zero we obtain:
2 sin ( 2 θ ) + 2 3 cos ( 2 θ ) = 0 ⟹ tan ( 2 θ ) = − 3 ⟹ 2 θ = 3 2 π ⟹ θ = 3 π
Replacing θ = 3 π in the above equation ⟹ 4 x ′ 2 − 4 y ′ = 0 ⟹ y ′ = x ′ 2
Similarly for − 2 x 2 − 4 3 x y − 6 y 2 + 2 3 x − 2 y + 9 = 0 we obtain y ′ = − 2 x ′ 2 + 4 9 .
Working from x ′ y ′ system and minimizing the distance O P we have:
D = d 2 = x ′ 2 + ( x ′ 2 − y 0 ′ ) 2 ⟹ d x ′ d D = x ′ ( 2 x ′ 2 − ( 2 y 0 ′ − 1 ) ) = 0 ⟹
x ′ = ± 2 2 y 0 ′ − 1 ⟹ 2 2 y 0 ′ − 1 + ( 2 2 y 0 ′ − 1 − y 0 ′ ) 2 = 1
⟹ 2 2 y 0 ′ − 1 = 4 3 ⟹ y 0 ′ = 4 5 ⟹ A : ( 0 , 4 9 ) and x ′ = ± 2 3
⟹ P : ( 2 3 , 0 ) and Q : ( − 2 3 , 0 ) and the equation of the circle in the x ′ y ′ system is
x ′ 2 + ( y ′ − 4 5 ) 2 = 1
For R 1 and R 2 :
I = 2 ∫ 0 2 3 ( − 2 x ′ 2 + 4 9 ) − ( 4 5 − 1 − x ′ 2 ) d x ′ =
2 ∫ 0 2 3 ( − 2 x ′ 2 + 1 + 1 − x ′ 2 ) d x ′ .
Letting x ′ = sin ( θ ) ⟹ d x ′ = cos ( θ ) ⟹
I = 2 ( ( − 3 2 x ′ 3 + x ′ ) ∣ 0 2 3 + ( 2 1 ( θ + 2 1 sin ( 2 θ ) ) ) ∣ 0 3 π ) =
2 ( 8 3 3 + 6 π ) = 4 3 3 + 3 π
⟹ A 1 = π − I = 3 2 π − 4 3 3
For R 3 :
For A 2 = 2 ∫ 0 2 3 ( ( 4 5 − 1 − x ′ 2 ) − x ′ 2 ) d x =
2 ∫ 0 2 3 ( 4 5 − x ′ 2 − 1 − x ′ 2 ) d x ′
Letting x ′ = sin ( θ ) ⟹ d x ′ = cos ( θ ) ⟹
A 2 = 2 ( 4 5 x ′ − 3 x ′ 3 ∣ 0 2 3 − 2 1 ( θ + 2 1 sin ( 2 θ ) ) ∣ 0 3 π ) =
2 ( 4 8 3 − 8 3 − 6 π ) = 4 3 3 − 3 π
A 2 = 4 3 3 − 3 π
⟹ the desired area A = A 1 + A 2 = 3 π ≈ 1 . 0 4 7 1 9 7 5 5