Still Not the Deathly Hallows Symbol

Geometry Level 5

As shown above, two identical ellipses are arranged in an equilateral triangle. There exists the unique value of eccentricity e e , such that:

  • The major axis of the vertically-positioned ellipse completely overlaps the height of the triangle.
  • The horizontally-positioned ellipse whose major axis is parallel to one of the three legs shares three points of tangency with the triangle and one point of tangency with another ellipse.

Input 1 0 6 e \lfloor 10^6 e\rfloor as your answer.


This problem is inspired by the comment (under Pi Han Goh 's solution) from the other problem I posted.


The answer is 968245.

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1 solution

Mark Hennings
Feb 16, 2021

Let the x x -axis be the base of the triangle. Then the horizontal ellipse will have equation x 2 a 2 + ( y b ) 2 b 2 = 1 \frac{x^2}{a^2} + \frac{(y-b)^2}{b^2} \; = \; 1 and this will be tangential to the slant lines y = 2 a ± x 3 y \; =\; 2a \pm x\sqrt{3} A standard result, effectively derived by @Pi Han Goh , states that a line y = m x + c y = mx + c will be tangential to the ellipse x 2 a 2 + y 2 b 2 = 1 \tfrac{x^2}{a^2} + \tfrac{y^2}{b^2} = 1 provided that a 2 m 2 + b 2 = c 2 a^2m^2 + b^2 = c^2 . In this case, we deduce that 3 a 2 + b 2 = ( 2 a b ) 2 3a^2 + b^2 \; = \; (2a - b)^2 and hence that a = 4 b a = 4b , which makes the eccentricity of the ellipse equal to e = 1 4 15 e=\tfrac14\sqrt{15} , giving 1 0 6 e = 968245 \lfloor 10^6e\rfloor = \boxed{968245} .

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