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Geometry Difficulty 5.4 AIME, harder Find the answer

In ABC,ω\triangle A B C, \omega is the circumcircle, II is the incenter and IAI_{A} is the AA-excenter. Let MM be the midpoint of arc BAC^\widehat{B A C} on ω\omega, and suppose that X,YX, Y are the projections of II onto MIAM I_{A} and IAI_{A} onto MIM I, respectively. If XYIA\triangle X Y I_{A} is an equilateral triangle with side length 1, compute the area of ABC\triangle A B C.

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

Solution

Using Fact 5, we know that IIAI I_{A} intersects the circle (ABC)(A B C) at MAM_{A}, which is the center of (IIABCXY)(I I_{A} B C X Y). Let RR be the radius of the latter circle. We have R=13R=\frac{1}{\sqrt{3}}. We have AIM=YIIA=YIX=π3\angle A I M=\angle Y I I_{A}=\angle Y I X=\frac{\pi}{3}. Also, IIAM=IMIA\angle I I_{A} M=\angle I M I_{A} by calculating the angles from the equilateral triangle. Using 90603090-60-30 triangles, we have: AI=12MI=12IIA=RA I=\frac{1}{2} M I=\frac{1}{2} I I_{A}=R, AM=32MI=3RA M=\frac{\sqrt{3}}{2} M I=\sqrt{3} R, MMA2=AM2+AMA2=7R2M M_{A}^{2}=A M^{2}+A M_{A}^{2}=7 R^{2}. Now, let JJ and NN be the feet of the altitudes from AA and BB respectively on MMAM M_{A}. Note that as MM is an arc midpoint of BC,NB C, N is actually the midpoint of BCB C. MAJ=AMA2MMA=47RM_{A} J=\frac{A M_{A}^{2}}{M M_{A}}=\frac{4}{\sqrt{7}} R, MAN=BMA2MMA=17RM_{A} N=\frac{B M_{A}^{2}}{M M_{A}}=\frac{1}{\sqrt{7}} R. Thus JN=37RJ N=\frac{3}{\sqrt{7}} R. Also, we have, BN2=MANMN=67R2B N^{2}=M_{A} N \cdot M N=\frac{6}{7} R^{2}. Now, [ABC]=12JNBC=JNBN=367R2=67[A B C]=\frac{1}{2} J N \cdot B C=J N \cdot B N=\frac{3 \sqrt{6}}{7} R^{2}=\frac{\sqrt{6}}{7}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.