Area Festival

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A parabola is tangent to the circle x 2 + y 2 = 1 x^2 + y^2 = 1 at ( 3 2 , 1 2 ) (\dfrac{\sqrt{3}}{2},\dfrac{1}{2}) and ( 3 2 , 1 2 ) (-\dfrac{\sqrt{3}}{2},\dfrac{1}{2}) as shown above.

If the area A A of the region R R bounded by the parabola and the circle above can be represented as A = a a b π a A = \dfrac{a\sqrt{a}}{b} - \dfrac{\pi}{a} , where a a and b b are coprime positive integers, find a + b a + b .


The answer is 7.

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1 solution

Rocco Dalto
Dec 3, 2019

m O A = 1 3 m = 3 m_{OA} = \dfrac{1}{\sqrt{3}} \implies m_{\perp} = -\sqrt{3}

m O B = 1 3 m = 3 m_{OB} = -\dfrac{1}{\sqrt{3}} \implies m^{*}_{\perp} = \sqrt{3}

Let y = a x 2 + b x + c d y d x = 2 a x + b y = ax^2 + bx + c \implies \dfrac{dy}{dx} = 2ax + b

d y d x ( x = 3 2 ) = 3 a + b = 3 \implies \dfrac{dy}{dx}|_{(x = \dfrac{\sqrt{3}}{2})} = \sqrt{3}a + b = -\sqrt{3}

and

d y d x ( x = 3 2 ) = 3 a + b = 3 \dfrac{dy}{dx}|_{(x = \dfrac{-\sqrt{3}}{2})} = -\sqrt{3}a + b = \sqrt{3}

Solving the above system we obtain a = 1 a = -1 and b = 0 b = 0 and using A ( 3 2 , 1 2 ) A(\dfrac{\sqrt{3}}{2},\dfrac{1}{2}) \implies

1 2 = 1 ( 3 2 ) 2 + c c = 5 4 \dfrac{1}{2} = -1(\dfrac{\sqrt{3}}{2})^2 + c \implies c = \dfrac{5}{4} \implies y = 5 4 x 2 y = \dfrac{5}{4} - x^2

and for the portion of the circle we have y = 1 x 2 y = \sqrt{1 - x^2}

A = 2 0 3 2 ( ( 5 4 x 2 ) 1 x 2 ) d x \implies A = 2\displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} ((\dfrac{5}{4} - x^2) - \sqrt{1 - x^2}) dx

= 2 ( 3 2 0 3 2 1 x 2 d x ) = 2(\dfrac{\sqrt{3}}{2} - \displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} \sqrt{1 - x^2} dx)

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

0 3 2 1 x 2 d x = 0 π 3 ( 1 + cos ( 2 θ ) ) d θ = \displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} \sqrt{1 - x^2} dx = \displaystyle\int_{0}^{\dfrac{\pi}{3}} (1 + \cos(2\theta)) d\theta = 1 2 ( θ + 1 2 sin ( 2 θ ) ) 0 π 3 = \dfrac{1}{2}(\theta + \dfrac{1}{2}\sin(2\theta))|_{0}^{\dfrac{\pi}{3}} =

1 2 ( π 3 + 3 4 ) \dfrac{1}{2}(\dfrac{\pi}{3} + \dfrac{\sqrt{3}}{4})

A = 3 ( π 3 + 3 4 ) = 3 3 4 π 3 \implies A = \sqrt{3} - (\dfrac{\pi}{3} + \dfrac{\sqrt{3}}{4}) = \dfrac{3\sqrt{3}}{4} - \dfrac{\pi}{3} = a a b π a a + b = 7 = \dfrac{a\sqrt{a}}{b} - \dfrac{\pi}{a} \implies a + b = \boxed{7} .

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