In △ABC,ω is the circumcircle, I is the incenter and IA is the A-excenter. Let M be the midpoint of arc BAC on ω, and suppose that X,Y are the projections of I onto MIA and IA onto MI, respectively. If △XYIA is an equilateral triangle with side length 1, compute the area of △ABC.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Using Fact 5, we know that IIA intersects the circle (ABC) at MA, which is the center of (IIABCXY). Let R be the radius of the latter circle. We have R=31. We have ∠AIM=∠YIIA=∠YIX=3π. Also, ∠IIAM=∠IMIA by calculating the angles from the equilateral triangle. Using 90−60−30 triangles, we have: AI=21MI=21IIA=R, AM=23MI=3R, MMA2=AM2+AMA2=7R2. Now, let J and N be the feet of the altitudes from A and B respectively on MMA. Note that as M is an arc midpoint of BC,N is actually the midpoint of BC. MAJ=MMAAMA2=74R, MAN=MMABMA2=71R. Thus JN=73R. Also, we have, BN2=MAN⋅MN=76R2. Now, [ABC]=21JN⋅BC=JN⋅BN=736R2=76.
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