Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Find the answer

Let ABCA B C be a triangle such that AB=13,BC=14,CA=15A B=13, B C=14, C A=15 and let E,FE, F be the feet of the altitudes from BB and CC, respectively. Let the circumcircle of triangle AEFA E F be ω\omega. We draw three lines, tangent to the circumcircle of triangle AEFA E F at A,EA, E, and FF. Compute the area of the triangle these three lines determine.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Note that AEFABCA E F \sim A B C. Let the vertices of the triangle whose area we wish to compute be P,Q,RP, Q, R, opposite A,E,FA, E, F respectively. Since H,OH, O are isogonal conjugates, line AHA H passes through the circumcenter of AEFA E F, so QRBCQ R \| B C. Let MM be the midpoint of BCB C. We claim that M=PM=P. This can be seen by angle chasing at E,FE, F to find that PFB=ABC,PEC=ACB\angle P F B=\angle A B C, \angle P E C=\angle A C B, and noting that MM is the circumcenter of BFECB F E C. So, the height from PP to QRQ R is the height from AA to BCB C, and thus if KK is the area of ABCA B C, the area we want is QRBCK\frac{Q R}{B C} K. Heron's formula gives K=84K=84, and similar triangles QAF,MBFQ A F, M B F and RAE,MCER A E, M C E give QA=BC2tanBtanAQ A=\frac{B C}{2} \frac{\tan B}{\tan A}, RA=BC2tanCtanAR A=\frac{B C}{2} \frac{\tan C}{\tan A}, so that QRBC=tanB+tanC2tanA=tanBtanC12=1110\frac{Q R}{B C}=\frac{\tan B+\tan C}{2 \tan A}=\frac{\tan B \tan C-1}{2}=\frac{11}{10}, since the height from AA to BCB C is 12 . So our answer is 4625\frac{462}{5}.

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