Elliptical Madness.

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Find the value of j j for which the distance c c from the center of the ellipse above to the focus is c = 120 3 c = \dfrac{120}{\sqrt{3}} .


The answer is 75.

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1 solution

Rocco Dalto
Sep 18, 2018

If the area A A of the ellipse above

(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

Let

x = x cos ( θ ) y sin ( θ ) x = x'\cos(\theta) - y'\sin(\theta)

y = x sin ( θ ) + y cos ( θ ) y = x'\sin(\theta) + y'\cos(\theta)

2 ( x 2 cos 2 ( θ ) sin ( 2 θ ) x y + y 2 sin 2 ( θ ) ) ( ( x 2 y 2 2 ) sin ( 2 θ ) + cos ( 2 θ ) x y ) 2(x'^2\cos^2(\theta) - \sin(2\theta)x'y' + y'^2\sin^2(\theta)) - ((\dfrac{x'^2 - y'^2}{2})\sin(2\theta) + \cos(2\theta)x'y') + 2 ( x 2 cos 2 ( θ ) ) + sin ( 2 θ ) x y + y 2 sin 2 ( θ ) ) 5 j ( x cos ( θ ) y sin ( θ ) ) + 2 j ( x sin ( θ ) + y cos ( θ ) ) = 0 + 2(x'^2\cos^2(\theta)) + \sin(2\theta)x'y' + y'^2\sin^2(\theta)) - 5j(x'\cos(\theta) - y'\sin(\theta)) + 2j(x'\sin(\theta) + y'\cos(\theta)) = 0

Setting x y x'y' terms to zero cos ( 2 θ ) = 0 θ = π 4 \implies \cos(2\theta) = 0 \implies \theta = \dfrac{\pi}{4}

\implies

x = 1 2 ( x y ) x = \dfrac{1}{\sqrt{2}}(x' - y')

y = 1 2 ( x + y ) y = \dfrac{1}{\sqrt{2}}(x' + y')

3 x 2 + 5 y 2 3 2 j x + 7 2 j y = 0 3 ( x j 2 ) 2 + 5 ( y + 7 5 2 j ) 2 = 32 j 2 5 \implies 3x'^2 + 5y'^2 - 3\sqrt{2}jx' + 7\sqrt{2}jy' = 0 \implies 3(x' - \dfrac{j}{\sqrt{2}})^2 + 5(y' + \dfrac{7}{5\sqrt{2}}j)^{2} = \dfrac{32j^{2}}{5}

( x j 2 ) 2 ( 32 j 2 15 ) + ( y + 7 5 2 j ) 2 ( 32 j 2 25 ) \implies \dfrac{(x' - \dfrac{j}{\sqrt{2}})^2}{(\dfrac{32j^2}{15})} + \dfrac{(y' + \dfrac{7}{5\sqrt{2}}j)^2}{(\dfrac{32j^{2}}{25})}

a = 4 2 j 15 \implies a = \dfrac{4\sqrt{2}j}{\sqrt{15}} and b = 4 2 j 5 c = a 2 b 2 = 8 5 3 j = 120 3 j = 75 b = \dfrac{4\sqrt{2}j}{5} \implies c = \sqrt{a^2 - b^2} = \dfrac{8}{5\sqrt{3}}j = \dfrac{120}{\sqrt{3}} \implies j = \boxed{75} .

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