Tangent Conics

Algebra Level 5

A parabola y = x 2 y=x^2 is drawn. An ellipse is drawn with one focus at the focus of the parabola and another focus on the line y = 2014 y=2014 . Given that the ellipse is tangent to the parabola at coordinate ( a , a 2 ) (a,a^2) , then find the maximum possible integer value of a a .

Details and Assumptions

  • The diagram above is not the correct solution (I just have trouble graphing tilted ellipses to my liking).


The answer is 44.

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1 solution

Daniel Liu
Aug 22, 2014

Suppose we start at the focus common to both the ellipse and the parabola, and shoot a point-like ball at the tangent point ( a , a 2 ) (a,a^2) . Because the point bounces off the parabola, it will travel vertically after reflection. However, since it also bounces off the ellipse, it will travel to the ellipse's second focus. Thus, we know that the ellipse's second focus's x-coordinate is equal to a a . Thus, a 2 2014 a 2014 44 a^2 \le 2014\implies a\le \sqrt{2014}\approx \boxed{44} and we are done.

Don't you think that it must have been decimal answer. I was getting 2014 \sqrt{2014} but confused where I am doing mistake. And btw, 2014 \sqrt{2014} is more close to 45!

Pranjal Jain - 6 years, 6 months ago

Genius solution. Thanks for this question as well as solution.

Rishik Jain - 4 years, 2 months ago

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