Hyperbola and Ellipse.

Geometry Level 5

In the above diagram, the ellipse goes thru the points Q , M , N , R , P Q,M,N,R,P and the hyperbola goes thru the points A , Q , B , P , C A,Q,B,P,C .

Find the area of the region R R bounded by the hyperbola and the ellipse above to five decimal places.


The answer is 44.91295.

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1 solution

Rocco Dalto
Sep 21, 2018

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

Using x 2 + B x y + C y 2 + D x + E y + F = 0 x^2 + Bxy + Cy^2 + Dx + Ey + F = 0 and the points I chose above to generate the hyperbola.

(1) ( 0 , 4 ) : 16 C 4 E + F = 0 F = 4 E 16 C (0,-4): 16C - 4E + F = 0 \implies \boxed{F = 4E - 16C}

(2) ( 8 , 4 ) : 4 B D = 8 D = 4 B 8 (8,-4): 4B - D = 8 \implies \boxed{D = 4B - 8} .

Using (1) and (2) above \implies

(3): ( 4 , 8 ) : 4 B + 12 C E = 12 (-4,-8): 4B + 12C - E = -12

(4) ( 12 , 8 ) : 12 B + 12 C E = 12 (12,-8): -12B + 12C - E = -12

(5) ( 4 , 2 ) : 4 B 6 C + E = 8 (4,-2): 4B - 6C + E = 8

(3) + (5) 4 B + 3 C = 2 \implies 4B + 3C =-2

(3) - (4) B = 0 C = 2 3 D = 8 E = 4 F = 80 3 \implies B = 0 \implies C = -\dfrac{2}{3} \implies D = -8 \implies E = 4 \implies F = \dfrac{80}{3}

3 x 2 2 y 2 24 x + 12 y = 80 3 x 2 + 2 y 2 + 24 x 12 y = 80 \implies 3x^2 - 2y^2 - 24x + 12y = -80 \implies -3x^2 + 2y^2 + 24x - 12y = 80 \implies

2 ( y 3 ) 2 3 ( x 4 ) 2 = 50 2(y - 3)^2 - 3(x - 4)^2 = 50

Using the given portion of hyperbola below the line x 8 4 \dfrac{x - 8}{4} \implies y h ( x ) = 3 1 2 ( 50 + 3 ( x 4 ) 3 A h = 0 8 y h ( x ) d x y_{h}(x) = 3 - \sqrt{\dfrac{1}{2}(50 + 3(x - 4)^3} \implies A_{h} = \displaystyle\int_{0}^{8} y_{h}(x) dx .

For I ( x ) = 1 2 50 + 3 ( x 4 ) 2 I(x) = \dfrac{1}{\sqrt{2}}\displaystyle\int \sqrt{50 + 3(x - 4)^2}

Let 3 ( x 4 ) = 50 tan ( θ ) d x = 50 3 sec 2 ( θ ) d θ \sqrt{3}(x - 4) = \sqrt{50}\tan(\theta) \implies dx = \sqrt{\dfrac{50}{3}}\sec^2(\theta) d\theta \implies I ( θ ) = 50 6 sec 3 ( θ ) d θ I(\theta) = \dfrac{50}{\sqrt{6}}\displaystyle\int \sec^3(\theta) d\theta

For sec 3 ( θ ) d θ \displaystyle\int \sec^3(\theta) d\theta

Let u = sec ( θ ) d u = sec ( θ ) tan ( θ ) u = \sec(\theta) \implies du = \sec(\theta)\tan(\theta) and d v = sec 2 ( θ ) v = tan ( θ ) dv = \sec^2(\theta) \implies v = \tan(\theta) \implies sec 3 ( θ ) d θ = sec ( θ ) tan ( θ ) sec ( θ ) + sec ( θ ) d θ ( sec 3 ( θ ) d θ = 1 2 ( sec ( θ ) tan ( θ ) + ln sec ( θ ) + tan ( θ ) ) \displaystyle\int \sec^3(\theta) d\theta = \sec(\theta)\tan(\theta) - \displaystyle\int \sec(\theta) + \displaystyle\int \sec(\theta) d\theta \implies (\displaystyle\int \sec^3(\theta) d\theta = \dfrac{1}{2}(\sec(\theta)\tan(\theta) + \ln|\sec(\theta) + \tan(\theta)|) \implies

I ( x ) 0 8 = 25 6 ( 50 + 3 ( x 4 ) 2 3 ( x 4 ) 50 + ln 50 + 3 ( x 4 ) 2 + 3 ( x 4 ) 50 ) 0 8 = 28 + 25 6 ln ( 7 2 + 4 3 7 2 4 3 ) I(x)|_{0}^{8} = \dfrac{25}{\sqrt{6}}(\dfrac{\sqrt{50 + 3(x - 4)^2}\sqrt{3}(x - 4)}{50} + \ln|\dfrac{\sqrt{50 + 3(x - 4)^2} + \sqrt{3}(x - 4)}{\sqrt{50}}|)|_{0}^{8} = 28 + \dfrac{25}{\sqrt{6}}\ln(\dfrac{7\sqrt{2} + 4\sqrt{3}}{7\sqrt{2} - 4\sqrt{3}})

A h = 4 25 6 ln ( 7 2 + 4 3 7 2 4 3 ) 21.69786 \implies A_{h} = -4 - \dfrac{25}{\sqrt{6}}\ln(\dfrac{7\sqrt{2} + 4\sqrt{3}}{7\sqrt{2} - 4\sqrt{3}}) \approx \boxed{-21.69786}

The area of the ellipse is A e = 0 8 y e ( x ) d x A_{e} = \displaystyle\int_{0}^{8} y_{e}(x) dx .

A e = 1 20 5 347 0 8 2207744 ( 347 x 1448 ) 2 + x 8 4 d x = A_{e} = \dfrac{1}{20}\sqrt{\dfrac{5}{347}}\displaystyle\int_{0}^{8} -\sqrt{2207744 - (347x - 1448)^{2}} + \dfrac{x - 8}{4} dx = 1 4 5 347 0 8 2207744 ( 347 x 1448 ) 2 + x 8 4 d x \dfrac{1}{4\sqrt{5}\sqrt{347}}\displaystyle\int_{0}^{8} -\sqrt{2207744 - (347x - 1448)^2} + \dfrac{x - 8}{4} 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 e = 275968 ( 347 ) 3 2 5 ( arcsin ( 347 x 1448 2207744 ) + 347 x 1448 2207744 2207744 ( 347 x 1448 ) 2 ) 0 8 = A_{e} = -\dfrac{275968}{(347)^{\frac{3}{2}}\sqrt{5}}(\arcsin(\dfrac{347x - 1448}{\sqrt{2207744}}) + \dfrac{347x - 1448}{2207744}\sqrt{2207744 - (347x - 1448)^2})|_{0}^{8} =

275968 ( 347 ) 3 2 5 ( arcsin ( 1328 2207744 ) + arcsin ( 1448 2207744 ) + 1328 444160 2207744 -\dfrac{275968}{(347)^{\frac{3}{2}}\sqrt{5}}(\arcsin(\dfrac{1328}{2207744}) + \arcsin(\dfrac{1448}{2207744}) + \dfrac{1328\sqrt{444160}}{2207744} + 1448 111040 2207744 ) 8 = \dfrac{1448\sqrt{111040}}{2207744}) - 8 =

275968 ( 347 ) 3 2 5 ( arcsin ( 83 28 11 ) + arcsin ( 181 56 11 ) + 513 34496 1735 ) 8 66.91295 -\dfrac{275968}{(347)^{\frac{3}{2}}\sqrt{5}}(\arcsin(\dfrac{83}{28\sqrt{11}}) + \arcsin(\dfrac{181}{56\sqrt{11}}) + \dfrac{513}{34496}\sqrt{1735}) - 8 \approx \boxed{-66.91295}

A = 0 8 y h ( x ) y e ( x ) d x = A h A e = 44.91295 \implies A = \displaystyle\int_{0}^{8} y_{h}(x) - y_{e}(x) dx = A_{h} - A_{e} = \boxed{44.91295} .

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