More of Ellipses 101.

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Let a > 1 a > 1 .

The length of the semi-major and semi-minor axis of the two congruent ellipses above are a a units and 1 1 unit respectively.

If the centers of the two ellipses are 1 1 unit apart and the area A A of the region R R can be expressed as A = a ( α π β β α ) A = a(\dfrac{\alpha\pi}{\beta} - \dfrac{\sqrt{\beta}}{\alpha}) , where α \alpha and β \beta are coprime positive integers, find α + β \alpha + \beta .


The answer is 5.

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1 solution

Rocco Dalto
Sep 26, 2018

I chose the ellipses ( x 2 a 2 + y 2 = 1 (\dfrac{x^2}{a^2} + y^2 = 1 above the line y = 0 ) y = 0) and ( x 2 a 2 + ( y 1 ) 2 = 1 (\dfrac{x^2}{a^2} + (y - 1)^2 = 1 below the line y = 1 ) y = 1) .

Solving for y y in both ellipses we want:

f ( x ) = 1 a a 2 x 2 f(x) = \dfrac{1}{a}\sqrt{a^2 - x^2} and g ( x ) = 1 1 a a 2 x 2 g(x) = 1 - \dfrac{1}{a}\sqrt{a^2 - x^2} .

f ( x ) = g ( x ) x = ± 3 2 a f(x) = g(x) \implies x = \pm\dfrac{\sqrt{3}}{2}a \implies the area A = 3 2 a 3 2 a f ( x ) g ( x ) d x = 2 a 3 2 a 3 3 2 a a 2 x 2 d x 3 a A = \displaystyle\int_{-\frac{\sqrt{3}}{2}a}^{\frac{\sqrt{3}}{2}a} f(x) - g(x) dx = \dfrac{2}{a}\displaystyle\int_{-\frac{\sqrt{3}}{2}a}^{\frac{3\sqrt{3}}{2}a} \sqrt{a^2 - x^2} dx - \sqrt{3}a

For I = 2 a 3 2 a 3 2 a a 2 x 2 d x I = \dfrac{2}{a}\displaystyle\int_{-\frac{\sqrt{3}}{2}a}^{\frac{\sqrt{3}}{2}a} \sqrt{a^2 - x^2} dx

Let x = a sin ( θ ) d x = a cos ( θ ) I = 2 a π 3 π 3 cos 2 ( θ ) d θ = a π 3 π 3 1 + cos ( 2 θ ) d θ = x = a\sin(\theta) \implies dx = a\cos(\theta) \implies I = 2a\displaystyle\int_{-\frac{\pi}{3}}^{\frac{\pi}{3}} \cos^2(\theta) d\theta = a\displaystyle\int_{-\frac{\pi}{3}}^{\frac{\pi}{3}} 1 + \cos(2\theta) d\theta = a ( θ + 1 2 sin ( 2 θ ) ) π 3 π 3 = a ( 2 π 3 + 3 2 ) A = a ( 2 π 3 3 2 ) = a ( α π β β α ) α + β = 5 a(\theta + \dfrac{1}{2}\sin(2\theta))|_{-\frac{\pi}{3}}^{\frac{\pi}{3}} = a(\dfrac{2\pi}{3} + \dfrac{\sqrt{3}}{2}) \implies A = a(\dfrac{2\pi}{3} - \dfrac{\sqrt{3}}{2}) = a(\dfrac{\alpha\pi}{\beta} - \dfrac{\sqrt{\beta}}{\alpha}) \implies \alpha + \beta = \boxed{5}

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