An Equal-Sums Locus

Problem

The ellipse G ⁣:x26+y2b2=1G \colon \dfrac{x^2}{6} + \dfrac{y^2}{b^2} = 1 (with 0<b<60 < b < \sqrt{6}) has foci F1,F2F_1, F_2 and minor-axis endpoints B1,B2B_1, B_2. A point PP on GG satisfies

PB1+PB2=PF1+PF2.|PB_1| + |PB_2| = |PF_1| + |PF_2|.

Determine, with proof, which of the following are true as bb varies:

1. the locus of such points PP is symmetric about the yy-axis; 2. for some bb, exactly two points PP of the ellipse satisfy the condition; 3. the minimum value of OP|OP| is 22.

Answer

Solution

Difficulty8/10
Topicsconic sections, AM-GM, analytic geometry, Symmetry

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