Ellipses and Circles.

Level 2

The ellipse and the circle above intersect at P P and Q Q .

Let A = A c A e A = A_{c} - A_{e} , where A c A_{c} and A e A_{e} are the areas of the circle and ellipse respectively.

Find the value of j j for which A = ( 625 1 5 3 2 18816 4 1 5 3 2 ) π A = (\dfrac{625 * 15^{\frac{3}{2}} - 18816}{4 * 15^{\frac{3}{2}}})\pi .


The answer is 7.

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1 solution

Rocco Dalto
Sep 20, 2018

For the ellipse:

(1) ( j , j ) : j 2 a + j 2 b + j 2 c + j d + j e = 0 (j,j): j^2a + j^2b + j^2c + jd + je = 0

(2) ( 0 , j ) : j c e = 0 c = e j (0,-j): jc - e = 0 \implies \boxed{c = \dfrac{e}{j}}

(3) 2 j , j ) : 4 j 2 a + 2 j 2 b + j 2 c + 2 j d + j e = 0 2j,j): 4j^2a + 2j^2b + j^2c + 2jd + je = 0

(4) ( 2 j , j ) : 4 j 2 a 2 j 2 b + j 2 c + 2 j d j e = 0 (2j,-j): 4j^2a - 2j^2b + j^2c + 2jd - je = 0

Subtracting (4) from (3) 2 j b + e = 0 b = e 2 j \implies 2jb + e = 0 \implies \boxed{b = -\dfrac{e}{2j}}

Replacing c = e j c = \dfrac{e}{j} and b = e 2 j b = -\dfrac{e}{2j} into equations (1) and (3) \implies

2 j a + 2 d = 3 e 2ja + 2d = -3e

4 j a + 2 d = e 4ja + 2d = -e

d = 5 2 e \implies \boxed{d = -\dfrac{5}{2}e} and a = e j \boxed{a = \dfrac{e}{j}}

1 j x 2 1 2 j x y + 1 j y 2 5 2 x + y = 0 \implies \dfrac{1}{j}x^2 - \dfrac{1}{2j}xy + \dfrac{1}{j}y^2 - \dfrac{5}{2}x + y = 0

2 x 2 x y + 2 y 2 5 j x + 2 j y = 0 \implies 2x^2 - xy + 2y^2 - 5jx + 2jy = 0

Solving for y y we obtain:

y = 1 4 ( x 2 j ± 1 15 384 j 2 ( 15 x 18 j ) 2 ) y = \dfrac{1}{4}(x - 2j \pm \dfrac{1}{\sqrt{15}}\sqrt{384j^2 - (15x - 18j)^2})

y = 1 4 ( x 2 j + 1 15 384 j 2 ( 15 x 18 j ) 2 ) y = \dfrac{1}{4}(x - 2j + \dfrac{1}{\sqrt{15}}\sqrt{384j^2 - (15x - 18j)^2}) for the portion of the ellipse above the line y = x 2 j 4 y = \dfrac{x - 2j}{4} .

Setting 384 j 2 ( 15 x 18 j ) 2 = 0 x = 2 15 ( 9 ± 4 6 ) j 384j^{2} - (15x - 18j)^{2} = 0 \implies x = \dfrac{2}{15}(9 \pm 4\sqrt{6})j are the points of intersection of the ellipse and the line y = x 2 j 4 y = \dfrac{x - 2j}{4} .

Letting a = 2 15 ( 9 4 6 ) j a = \dfrac{2}{15}(9 - 4\sqrt{6})j and b = 2 15 ( 9 + 4 6 ) j b = \dfrac{2}{15}(9 + 4\sqrt{6})j the area of the ellipse is A e = 2 4 15 a b 384 j 2 ( 15 x 18 j ) 2 ) A_{e} = \dfrac{2}{4\sqrt{15}}\displaystyle\int_{a}^{b} \sqrt{384j^{2} - (15x - 18j)^{2}})

Letting 15 x 18 j = 384 j sin ( θ ) d x = 384 15 j cos ( θ ) 15x - 18j = \sqrt{384}j\sin(\theta) \implies dx = \dfrac{\sqrt{384}}{15}j\cos(\theta) \implies

I ( θ ) = 384 15 j 2 cos 2 ( θ ) d θ = 384 30 j 2 ( 1 + cos ( 2 θ ) ) d θ = 192 15 j 2 ( θ + sin ( θ ) cos ( θ ) ) I(\theta) = \dfrac{384}{15}j^2\displaystyle\int \cos^2(\theta) d\theta = \dfrac{384}{30}j^2\displaystyle\int(1 + \cos(2\theta)) d\theta = \dfrac{192}{15}j^2(\theta + \sin(\theta)\cos(\theta))

A e = 96 1 5 3 2 j 2 ( arcsin ( 15 x 18 j 384 ) + ( 15 x 18 j ) 384 j 2 ( 15 x 18 j ) 2 ) 384 ) a b = \implies A_{e} = \dfrac{96}{15^{\frac{3}{2}}}j^2(\arcsin(\dfrac{15x - 18j}{\sqrt{384}}) + \dfrac{(15x - 18j)\sqrt{384j^2 - (15x - 18j)^2})}{384})|_{a}^{b} = 96 π 1 5 3 2 j 2 \boxed{\dfrac{96\pi}{15^{\frac{3}{2}}}j^2} .

For the circle ( x x 0 ) 2 + ( y y 0 ) 2 = r 2 (x- x_{0})^2 + (y - y_{0})^2 = r^2 :

( 0 , 0 ) : x 0 2 + y 0 2 = r 2 (0,0): x_{0}^2 + y_{0}^2 = r^2

( 0 , j ) : x 0 2 + j 2 + 2 j y 0 + y 0 2 = r 2 j ( j + 2 y 0 ) , j 0 y 0 = j 2 (0,-j): x_{0}^2 + j^2 + 2jy_{0} + y_{0}^2 = r^2 \implies j(j + 2y_{0}), \:\ j \neq 0 \implies y_{0} = -\dfrac{j}{2}

( 3 ( j + 1 ) , 0 ) : 9 ( j + 1 ) 2 + 6 ( j + 1 ) x 0 + x 0 2 + y 0 2 = r 2 ( j + 1 ) ( 3 ( j + 1 ) + 2 x 0 ) , j 1 x 0 = 3 2 ( j + 1 ) (-3(j + 1),0): 9(j + 1)^2 + 6(j+ 1)x_{0} + x_{0}^2 + y_{0}^2 = r^2 \implies (j + 1)(3(j + 1) + 2x_{0}), \:\ j \neq -1 \implies x_{0} = -\dfrac{3}{2}(j + 1)

r 2 = 10 j 2 + 18 j + 9 4 \implies r^2 = \dfrac{10j^2 + 18j + 9}{4}

A c = ( 10 j 2 + 18 j + 9 4 ) π \implies \boxed{A_{c} = (\dfrac{10j^2 + 18j + 9}{4})\pi}

A = A c A e = ( 10 j 2 + 18 j + 9 4 96 1 5 3 2 j 2 ) π = \implies A = A_{c} - A_{e} = (\dfrac{10j^2 + 18j + 9}{4} - \dfrac{96}{15^{\frac{3}{2}}}j^2)\pi = ( 2 ( 5 1 5 3 2 192 ) j 2 + 18 1 5 3 2 + 9 1 5 3 2 4 1 5 3 2 ) π = ( 625 1 5 3 2 18816 4 1 5 3 2 ) π (\dfrac{2(5 * 15^{\frac{3}{2}} - 192)j^2 + 18 * 15^{\frac{3}{2}} + 9 * 15^{\frac{3}{2}}}{4 * 15^{\frac{3}{2}}})\pi = (\dfrac{625 * 15^{\frac{3}{2}} - 18816}{4 * 15^{\frac{3}{2}}})\pi

( 150 15 384 ) j 2 + 270 15 j + 18816 9240 15 = 0 \implies (150\sqrt{15} - 384)j^2 + 270\sqrt{15}j + 18816 - 9240\sqrt{15} = 0

Using the quadratic formula and dropping the negative root j = 7 \implies j = \boxed{7} .

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