Maths Olympiad Prep

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Geometry Difficulty 5.3 AIME, harder Prove it United States

Problem:

In the quadrilateral MAREM A R E inscribed in a unit circle ω\omega, AMA M is a diameter of ω\omega, and EE lies on the angle bisector of RAM\angle R A M. Given that triangles RAMR A M and REMR E M have the same area, find the area of quadrilateral MAREM A R E.

Solution

Solution:

Since AEA E bisects RAM\angle R A M, we have RE=EMR E = E M, and E,AE, A lie on different sides of RMR M. Since AMA M is a diameter, ARM=90\angle A R M = 90^{\circ}. If the midpoint of RMR M is NN, then from [RAM]=[REM][R A M] = [R E M] and ARM=90\angle A R M = 90^{\circ}, we find AR=NEA R = N E. Note that OO, the center of ω\omega, NN, and EE are collinear, and by similarity of triangles NOMN O M and RAMR A M, ON=12AR=12NEO N = \frac{1}{2} A R = \frac{1}{2} N E. Therefore, ON=13O N = \frac{1}{3} and NE=23N E = \frac{2}{3}. By the Pythagorean theorem on triangle RAMR A M, RM=423R M = \frac{4 \sqrt{2}}{3}. Therefore, the area of MAREM A R E is 21242323=8292 \cdot \frac{1}{2} \cdot \frac{4 \sqrt{2}}{3} \cdot \frac{2}{3} = \frac{8 \sqrt{2}}{9}.

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