Ellipse Mania!

Geometry Level 4

If the area A A of the ellipse above can be represented as A = α π β 3 2 λ A = \dfrac{\alpha\pi}{\beta^{\frac{3}{2}}\sqrt{\lambda}} , where α , β \alpha, \beta and λ \lambda are coprime positive integers, find α + β + λ \alpha + \beta + \lambda .


The answer is 552288.

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2 solutions

First we set a coordinate system as depicted in the figure: As we can see here the equation of the conic containing five points ( x i , y i ) i = 1 , 2 , 3 , 4 , 5 {{\left( {{x}_{i}},{{y}_{i}} \right)}_{i=1,2,3,4,5}} , is

det ( x 2 x y y 2 x y 1 x 1 2 x 1 y 1 y 1 2 x 1 y 1 1 x 2 2 x 2 y 2 y 2 2 x 2 y 2 1 x 3 2 x 3 y 3 y 3 2 x 3 y 3 1 x 4 2 x 4 y 4 y 4 2 x 4 y 4 1 x 5 2 x 5 y 5 y 5 2 x 5 y 5 1 ) = 0 \det \left( \begin{matrix} {{x}^{2}} & xy & {{y}^{2}} & x & y & 1 \\ {{x}_{1}}^{2} & {{x}_{1}}{{y}_{1}} & {{y}_{1}}^{2} & {{x}_{1}} & {{y}_{1}} & 1 \\ {{x}_{2}}^{2} & {{x}_{2}}{{y}_{2}} & {{y}_{2}}^{2} & {{x}_{2}} & {{y}_{2}} & 1 \\ {{x}_{3}}^{2} & {{x}_{3}}{{y}_{3}} & {{y}_{3}}^{2} & {{x}_{3}} & {{y}_{3}} & 1 \\ {{x}_{4}}^{2} & {{x}_{4}}{{y}_{4}} & {{y}_{4}}^{2} & {{x}_{4}} & {{y}_{4}} & 1 \\ {{x}_{5}}^{2} & {{x}_{5}}{{y}_{5}} & {{y}_{5}}^{2} & {{x}_{5}} & {{y}_{5}} & 1 \\ \end{matrix} \right)=0 In our case, using the points A A , B B , C C , D D and F F , the equation of the ellipse is

det ( x 2 x y y 2 x y 1 25 0 0 5 0 1 9 0 0 3 0 1 16 0 0 4 0 1 0 0 144 0 12 1 9 24 64 3 8 1 ) = 0 44 x 2 5 x y + 10 y 2 + 88 x 65 y 660 = 0 \det \left( \begin{matrix} {{x}^{2}} & xy & {{y}^{2}} & x & y & 1 \\ 25 & 0 & 0 & -5 & 0 & 1 \\ 9 & 0 & 0 & 3 & 0 & 1 \\ 16 & 0 & 0 & -4 & 0 & 1 \\ 0 & 0 & 144 & 0 & 12 & 1 \\ 9 & 24 & 64 & 3 & 8 & 1 \\ \end{matrix} \right)=0\Leftrightarrow 44{{x}^{2}}-5xy+10{{y}^{2}}+88x-65y-660=0 The later is of the form
a x 2 + 2 b x y + c y 2 + 2 d x + 2 f y + g = 0 a{{x}^{2}}+2bxy+c{{y}^{2}}+2dx+2fy+g=0 with a = 44 b = 5 2 c = 10 d = 44 f = 65 2 g = 660 a=44 \ \ \ b=\dfrac{-5}{2} \ \ \ c=10 \ \ \ d=44 \ \ \ f=\dfrac{-65}{2} \ \ \ g=-660

The corresponding formulae for the semi-axes a , b {a}',{b}' of the ellipse are a , b = 2 ( a f 2 + c d 2 + g b 2 2 b d f a c g ) ± ( b 2 a c ) [ ( a c ) 2 + 4 b 2 ( a + c ) ] {a}',{b}'=\sqrt{\dfrac{2\left( a{{f}^{2}}+c{{d}^{2}}+g{{b}^{2}}-2bdf-acg \right)}{\pm \left( {{b}^{2}}-ac \right)\left[ \sqrt{{{\left( a-c \right)}^{2}}+4{{b}^{2}}}-\left( a+c \right) \right]}} Taking their product and applying the numerical values we have a b = a f 2 + c d 2 + g b 2 2 b d f a c g ( a c b 2 ) 3 2 = 44 × ( 65 2 ) 2 + 10 × 44 2 + ( 660 ) ( 5 2 ) 2 2 × ( 5 2 ) × 44 × ( 65 2 ) 44 × 10 × ( 660 ) ( 44 × 10 ( 5 2 ) 2 ) 3 2 = 551936 347 1735 = 551936 347 347 × 5 = 551936 347 3 2 5 \begin{aligned} {a}'{b}' & =\dfrac{a{{f}^{2}}+c{{d}^{2}}+g{{b}^{2}}-2bdf-acg}{{{\left( ac-{{b}^{2}} \right)}^{\frac{3}{2}}}} \\ & =\dfrac{44\times {{\left( -\dfrac{65}{2} \right)}^{2}}+10\times {{44}^{2}}+\left( -660 \right){{\left( -\dfrac{5}{2} \right)}^{2}}-2\times \left( -\dfrac{5}{2} \right)\times 44\times \left( -\dfrac{65}{2} \right)-44\times 10\times \left( -660 \right)}{{{\left( 44\times 10-{{\left( -\dfrac{5}{2} \right)}^{2}} \right)}^{\frac{3}{2}}}} \\ & =\dfrac{551936}{347\sqrt{1735}} \\ & =\dfrac{551936}{347\sqrt{347\times 5}} \\ & =\dfrac{551936}{{{347}^{\frac{3}{2}}}\sqrt{5}} \\ \end{aligned} Consequently, the area of the ellipse is

A = a b π = 551936 π 347 3 2 5 A={a}'{b}'\pi =\dfrac{551936\pi }{{{347}^{\frac{3}{2}}}\sqrt{5}} For the answer, α = 551936 \alpha =551936 , β = 347 \beta =347 , λ = 5 \lambda=5 , thus, α + β + λ = 552288 \alpha+\beta+\lambda=\boxed{552288} .

Rocco Dalto
Jan 10, 2021

Using a x 2 + b x y + c y 2 + d x + e y = 0 ax^2 + bxy + cy^2 + dx + ey = 0 and the points I chose above to generate the ellipse.

(1) ( 0 , 4 ) : 4 c e = 0 c = e 4 (0,-4): 4c - e = 0 \implies \boxed{c = \dfrac{e}{4}}

(2) ( 8 , 4 ) : 16 a + 8 b + 4 c + 2 d + e = 0 (8,4): 16a + 8b + 4c + 2d + e = 0

(3) ( 8 , 4 ) : 16 a 8 b + 4 c + 2 d e = 0 (8,-4): 16a - 8b + 4c + 2d - e = 0

Subtracting (3) from (2) we obtain: 8 b + e = 0 b = e 8 8b + e = 0 \implies \boxed{b = -\dfrac{e}{8}}

(4) ( 5 , 8 ) : 25 a + 40 b + 64 c + 5 d + 8 c = 0 (5,8): 25a + 40b + 64c + 5d + 8c = 0

Replacing c = e 4 c = \dfrac{e}{4} and b = e 8 b = -\dfrac{e}{8} into (2) and (4) \implies

16 a + 2 d = e 16a + 2d = -e

25 a + 5 d = 19 e 25a + 5d = -19e

a = 11 e 10 \implies \boxed{a = \dfrac{11e}{10}} and d = 93 e 10 \boxed{d = -\dfrac{93e}{10}}

11 10 x 2 1 8 x y + 1 4 y 2 93 10 x + y = 0 \implies \dfrac{11}{10}x^2 - \dfrac{1}{8}xy + \dfrac{1}{4}y^2 - \dfrac{93}{10}x + y = 0 \implies 44 x 2 5 x y + 10 y 2 372 x + 40 y = 0 10 y 2 + 5 ( x 8 ) y + 44 x 2 372 x = 0 44x^2 - 5xy + 10y^2 - 372x + 40y = 0 \implies 10y^2 + 5(x - 8)y + 44x^2 - 372x = 0

Solving for y y we obtain:

y = x 8 4 ± 1 20 5 347 2207744 ( 347 x 1448 ) 2 y = \dfrac{x - 8}{4} \pm \dfrac{1}{20}\sqrt{\dfrac{5}{347}}\sqrt{2207744 - (347x - 1448)^{2}} .

y ( x ) = x 8 4 + 1 20 5 347 2207744 ( 347 x 1448 ) 2 y(x) = \dfrac{x - 8}{4} +\dfrac{1}{20}\sqrt{\dfrac{5}{347}}\sqrt{2207744 - (347x - 1448)^2} for the portion of the ellipse above the line y = x 8 4 y = \dfrac{x - 8}{4} .

Setting 2207744 ( 347 x 1448 ) 2 = 0 x = 1448 ± 448 11 347 2207744 - (347x - 1448)^2 = 0 \implies x = \dfrac{1448 \pm 448\sqrt{11}}{347} are the points of intersection of the ellipse and the line x 8 4 \dfrac{x - 8}{4} .

Letting a = 1448 448 11 347 a = \dfrac{1448 - 448\sqrt{11}}{347} and b = 1448 + 448 11 347 b =\dfrac{1448 + 448\sqrt{11}}{347} the area of the ellipse is A = 2 a b y ( x ) x 8 4 d x = A = 2\displaystyle\int_{a}^{b} y(x) - \dfrac{x - 8}{4} \:\ dx =
1 10 5 347 a b 2207744 ( 347 x 1448 ) 2 d x = \dfrac{1}{10}\sqrt{\dfrac{5}{347}}\displaystyle\int_{a}^{b} \sqrt{2207744 - (347x - 1448)^{2}} dx = 1 2 5 347 a b 2207744 ( 347 x 1448 ) 2 d x \dfrac{1}{2\sqrt{5}\sqrt{347}}\displaystyle\int_{a}^{b} \sqrt{2207744 - (347x - 1448)^2} dx .

For I ( x ) = 2207744 ( 347 x 1448 ) 2 d x I(x) = \displaystyle\int \sqrt{2207744 - (347x - 1448)^2} dx

Let 347 x 1448 = 2207744 sin ( θ ) d x = 2207744 347 cos ( θ ) 347x - 1448 = \sqrt{2207744}\sin(\theta) \implies dx = \dfrac{\sqrt{2207744}}{347}\cos(\theta)

I ( θ ) = 2207744 347 cos 2 ( θ ) d θ = 2207744 694 ( 1 + cos ( 2 θ ) ) d θ = 2207744 694 ( θ + sin ( θ ) cos ( θ ) ) \implies I(\theta) = \dfrac{2207744}{347}\displaystyle\int \cos^2(\theta) d\theta = \dfrac{2207744}{694}\displaystyle\int (1 + \cos(2\theta)) d\theta = \dfrac{2207744}{694}(\theta +\sin(\theta)\cos(\theta))

I ( x ) = 2207744 694 ( arcsin ( 347 x 1448 2207744 ) + 347 x 1448 2207744 2207744 ( 347 x 1448 ) 2 ) \implies I(x) = \dfrac{2207744}{694}(\arcsin(\dfrac{347x - 1448}{\sqrt{2207744}}) + \dfrac{347x - 1448}{2207744}\sqrt{2207744 - (347x - 1448)^2}) \implies

A = 551936 ( 347 ) 3 2 5 ( arcsin ( 347 x 1448 2207744 ) + 347 x 1448 2207744 2207744 ( 347 x 1448 ) 2 ) a b = 1103872 ( 347 ) 3 2 5 ( π 2 ) = A = \dfrac{551936}{(347)^{\frac{3}{2}}\sqrt{5}}(\arcsin(\dfrac{347x - 1448}{\sqrt{2207744}}) + \dfrac{347x - 1448}{2207744}\sqrt{2207744 - (347x - 1448)^2})|_{a}^{b} = \dfrac{1103872}{(347)^{\frac{3}{2}}\sqrt{5}}(\dfrac{\pi}{2}) =

551936 π ( 347 ) 3 2 5 = α π β 3 2 λ \dfrac{551936\pi}{(347)^{\frac{3}{2}}\sqrt{5}} = \dfrac{\alpha\pi}{\beta^{\frac{3}{2}}\sqrt{\lambda}} α + β + λ = 552288 \implies \alpha + \beta + \lambda = \boxed{552288} .

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