A Dot-Product Condition on an Ellipse

Problem

In the coordinate plane, consider the ellipse Γ ⁣:x2a2+y2b2=1\Gamma \colon \dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1 (with a>b>0a > b > 0). Let AA be an endpoint of the major axis, BB an endpoint of the minor axis, and FF a focus of Γ\Gamma. Suppose there exist points P,QP, Q on Γ\Gamma, symmetric about the origin OO, such that

FPFQ+FAFB=AB2.\overrightarrow{FP} \cdot \overrightarrow{FQ} + \overrightarrow{FA} \cdot \overrightarrow{FB} = |AB|^2.

1. Prove that the focus FF lies on the extension of segment AOAO beyond OO. 2. Find the range of possible eccentricities of Γ\Gamma.

Answer

Solution

Difficulty7/10
TopicsQuadratic Equations, conic sections, analytic geometry, Symmetry, vectors

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