No Einstein here! Only an innocent conic

Geometry Level 5

The equation of an ellipse is given by 14 x 2 4 x y + 11 y 2 44 x 58 y + 71 = 0 14x^2 - 4xy + 11y^2 -44x -58y + 71 =0 . If the slope of the major axis of this ellipse is m m and the sum of distances of any variable point on the ellipse from its focii is c c , evaluate the value of m c 2 mc^2 .


The answer is 48.

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1 solution

As we can see here , the ellipse with implicit equation A x 2 + B x y + C y 2 + D x + E y + F = 0 A{{x}^{2}}+Bxy+C{{y}^{2}}+Dx+Ey+F=0 is an isometric transformation (translation and rotation) of the ellipse with canonical equation x 2 a 2 + y 2 b 2 = 1 \dfrac{{{x}^{2}}}{{{a}^{2}}}+\dfrac{{{y}^{2}}}{{{b}^{2}}}=1 .

For the rotation angle, Θ \Theta , it holds the relation Θ = arctan ( 1 B ( C A ( A C ) 2 + B 2 ) ) . \Theta =\arctan \left( \frac{1}{B}\left( C-A-\sqrt{{{\left( A-C \right)}^{2}}+{{B}^{2}}} \right) \right). Hence, the slope of the major axis is m = tan Θ = 1 B ( C A ( A C ) 2 + B 2 ) m=\tan \Theta =\dfrac{1}{B}\left( C-A-\sqrt{{{\left( A-C \right)}^{2}}+{{B}^{2}}} \right) .
Using A = 14 , B = 4 , C = 11 , D = 44 , E = 58 , F = 71 A=14,\text{ }B=-4,\text{ }C=11,\text{ }D=-44,\text{ }E=-58,\text{ }F=71 , we find m = 2 m=2 .

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Furthermore, a = 2 ( A E 2 + C D 2 B D E + ( B 2 4 A C ) F ) ( ( A + C ) + ( A C ) 2 + B 2 ) B 2 4 A C = 6 a=\dfrac{-\sqrt{2\left( A{{E}^{2}}+C{{D}^{2}}-BDE+\left( {{B}^{2}}-4AC \right)F \right)\left( \left( A+C \right)+\sqrt{{{\left( A-C \right)}^{2}}+{{B}^{2}}} \right)}}{{{B}^{2}}-4AC}=\sqrt{6}

Thus, the sum of distances of any variable point on the ellipse from its focii is c = 2 a = 2 6 c=2a=2\sqrt{6} .

For the answer, m c 2 = 2 × ( 2 6 ) 2 = 48 m{{c}^{2}}=2\times {{\left( 2\sqrt{6} \right)}^{2}}=\boxed{48} .

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