Let ω1 be a circle of radius 5, and let ω2 be a circle of radius 2 whose center lies on ω1. Let the two circles intersect at A and B, and let the tangents to ω2 at A and B intersect at P. If the area of △ABP can be expressed as cab, where b is square-free and a,c are relatively prime positive integers, compute 100a+10b+c.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let O1 and O2 be the centers of ω1 and ω2, respectively. Because ∠O2AP+∠O2BP=90∘+90∘=180∘ quadrilateral O2APB is cyclic. But O2,A, and B lie on ω1, so P lies on ω1 and O2P is a diameter of ω1. From the Pythagorean theorem on triangle PAP2, we can calculate AP=46, so sin∠AOP=526 and cos∠AOP=51. Because △AOP and △BOP are congruent, we have sin∠APB=sin2∠AOP=2sin∠AOPcos∠AOP=2546 implying that [APB]=2PA⋅PBsin∠APB=251926
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