Maths Olympiad Prep

Library / /699 of 860

Geometry Difficulty 5.4 AIME, harder Find the answer

ABCA B C is an acute triangle with incircle ω\omega. ω\omega is tangent to sides BC,CA\overline{B C}, \overline{C A}, and AB\overline{A B} at D,ED, E, and FF respectively. PP is a point on the altitude from AA such that Γ\Gamma, the circle with diameter AP\overline{A P}, is tangent to ω\omega. Γ\Gamma intersects AC\overline{A C} and AB\overline{A B} at XX and YY respectively. Given XY=8,AE=15X Y=8, A E=15, and that the radius of Γ\Gamma is 5, compute BDDCB D \cdot D C.

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

Solution

By the Law of Sines we have sinA=XYAP=45\sin \angle A=\frac{X Y}{A P}=\frac{4}{5}. Let I,TI, T, and QQ denote the center of ω\omega, the point of tangency between ω\omega and Γ\Gamma, and the center of Γ\Gamma respectively. Since we are told ABCA B C is acute, we can compute tanA2=12\tan \angle \frac{A}{2}=\frac{1}{2}. Since EAI=A2\angle E A I=\frac{A}{2} and AE\overline{A E} is tangent to ω\omega, we find r=AE2=152r=\frac{A E}{2}=\frac{15}{2}. Let HH be the foot of the altitude from AA to BC\overline{B C}. Define hTh_{T} to be the homothety about TT which sends Γ\Gamma to ω\omega. We have hT(AQ)=DIh_{T}(\overline{A Q})=\overline{D I}, and conclude that A,TA, T, and DD are collinear. Now since AP\overline{A P} is a diameter of Γ,PAT\Gamma, \angle P A T is right, implying that DTHPD T H P is cyclic. Invoking Power of a Point twice, we have 225=AE2=ATAD=APAH225=A E^{2}=A T \cdot A D=A P \cdot A H. Because we are given radius of Γ\Gamma we can find AP=10A P=10 and AH=452=haA H=\frac{45}{2}=h_{a}. If we write a,b,c,sa, b, c, s in the usual manner with respect to triangle ABCA B C, we seek BDDC=(sb)(sc)B D \cdot D C=(s-b)(s-c). But recall that Heron's formula gives us s(sa)(sb)(sc)=K\sqrt{s(s-a)(s-b)(s-c)}=K where KK is the area of triangle ABCA B C. Writing K=rsK=r s, we have (sb)(sc)=r2ssa(s-b)(s-c)=\frac{r^{2} s}{s-a}. Knowing r=152r=\frac{15}{2}, we need only compute the ratio sa\frac{s}{a}. By writing K=12aha=rsK=\frac{1}{2} a h_{a}=r s, we find sa=ha2r=32\frac{s}{a}=\frac{h_{a}}{2 r}=\frac{3}{2}. Now we compute our answer, r2ssa=(152)2sasa1=6754\frac{r^{2} s}{s-a}=\left(\frac{15}{2}\right)^{2} \cdot \frac{\frac{s}{a}}{\frac{s}{a}-1}=\frac{675}{4}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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