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Geometry Difficulty 5.6 AIME, harder Find the answer

Triangle ABCA B C has incircle ω\omega which touches ABA B at C1,BCC_{1}, B C at A1A_{1}, and CAC A at B1B_{1}. Let A2A_{2} be the reflection of A1A_{1} over the midpoint of BCB C, and define B2B_{2} and C2C_{2} similarly. Let A3A_{3} be the intersection of AA2A A_{2} with ω\omega that is closer to AA, and define B3B_{3} and C3C_{3} similarly. If AB=9,BC=10A B=9, B C=10, and CA=13C A=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 A2A_{2} is the point of tangency of the excircle opposite AA to BCB C. Therefore, by considering the homothety centered at AA taking the excircle to the incircle, we notice that A3A_{3} is the intersection of ω\omega and the tangent line parallel to BCB C. It follows that A1B1C1A_{1} B_{1} C_{1} is congruent to A3B3C3A_{3} B_{3} C_{3} by reflecting through the center of ω\omega. We therefore need only find \left[A_{1} B_{1} C_{1}\right] /[A B C]. Since [A1BC1][ABC]=A1BBC1ABBC=((9+1013)/2)2910=110\frac{\left[A_{1} B C_{1}\right]}{[A B C]}=\frac{A_{1} B \cdot B C_{1}}{A B \cdot B C}=\frac{((9+10-13) / 2)^{2}}{9 \cdot 10}=\frac{1}{10} 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 [A3B3C3][ABC]=111049130413=1465\frac{\left[A_{3} B_{3} C_{3}\right]}{[A B C]}=1-\frac{1}{10}-\frac{49}{130}-\frac{4}{13}=\frac{14}{65}

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