A Fixed Point from Tangents to a Shrinking Circle

Problem

figure

As shown in the figure, let AA be the top vertex of the ellipse x24+y2=1\dfrac{x^2}{4} + y^2 = 1. The two lines through AA tangent to the circle M:(x+1)2+y2=r2M: (x+1)^2 + y^2 = r^2 (where 0<r<10 < r < 1) meet the ellipse again at points BB and DD, both different from AA. As rr varies, does the line BDBD always pass through a fixed point? If so, find this fixed point; if not, explain why.

Answer

Solution

Difficulty7/10
TopicsTangent Circle, conic sections, analytic geometry, Vieta's Formulas

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