Triangle ABC has incircle ω which touches AB at C1,BC at A1, and CA at B1. Let A2 be the reflection of A1 over the midpoint of BC, and define B2 and C2 similarly. Let A3 be the intersection of AA2 with ω that is closer to A, and define B3 and C3 similarly. If AB=9,BC=10, and CA=13, find \left[A_{3} B_{3} C_{3}\right] /[A B C].
A number or a short expression. Spacing and $ signs are ignored.
Solution
Notice that A2 is the point of tangency of the excircle opposite A to BC. Therefore, by considering the homothety centered at A taking the excircle to the incircle, we notice that A3 is the intersection of ω and the tangent line parallel to BC. It follows that A1B1C1 is congruent to A3B3C3 by reflecting through the center of ω. We therefore need only find \left[A_{1} B_{1} C_{1}\right] /[A B C]. Since [ABC][A1BC1]=AB⋅BCA1B⋅BC1=9⋅10((9+10−13)/2)2=101 and likewise \left[A_{1} B_{1} C\right] /[A B C]=49 / 130 and \left[A B_{1} C_{1}\right] /[A B C]=4 / 13, we get that [ABC][A3B3C3]=1−101−13049−134=6514
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