2013 AIME I Problem 13

Below is the professionally curated solution for Problem 13 of the 2013 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AIME I solutions, or check the answer key.

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Concepts:similaritygeometric sequenceHeron’s Formularecursion

Difficulty rating: 3160

13.

Triangle AB0C0AB_0C_0 has side lengths AB0=12,AB_0 = 12, B0C0=17,B_0C_0 = 17, and C0A=25.C_0A = 25. For each positive integer n,n, points BnB_n and CnC_n are located on ABn1\overline{AB_{n-1}} and ACn1,\overline{AC_{n-1}}, respectively, creating three similar triangles ABnCnBn1CnCn1ABn1Cn1.\triangle AB_nC_n \sim \triangle B_{n-1}C_nC_{n-1} \sim \triangle AB_{n-1}C_{n-1}. The area of the union of all triangles Bn1CnBnB_{n-1}C_nB_n for n1n \ge 1 can be expressed as pq,\frac{p}{q}, where pp and qq are relatively prime positive integers. Find q.q.

Solution:

By Heron's formula with s=27,s = 27, the area of AB0C0\triangle AB_0C_0 is 2715102=90.\sqrt{27 \cdot 15 \cdot 10 \cdot 2} = 90. In the similarity B0C1C0AB0C0,\triangle B_0C_1C_0 \sim \triangle AB_0C_0, side B0C0B_0C_0 corresponds to AC0,AC_0, so the ratio is r=1725,r = \frac{17}{25}, and C1C0C_1C_0 (corresponding to B0C0B_0C_0) equals 17r.17r. Hence AC1AC0=2517r25=1r2,\frac{AC_1}{AC_0} = \frac{25 - 17r}{25} = 1 - r^2, which is the similarity ratio of AB1C1\triangle AB_1C_1 to AB0C0.\triangle AB_0C_0.

Segments B1C1\overline{B_1C_1} and B0C1\overline{B_0C_1} split AB0C0\triangle AB_0C_0 into the three pieces, so [B0C1B1]=90(1r2(1r2)2)=90r2(1r2).[B_0C_1B_1] = 90\left(1 - r^2 - (1 - r^2)^2\right) = 90\,r^2(1 - r^2). Each successive stage repeats the construction inside ABnCn,\triangle AB_nC_n, scaling all areas by (1r2)2,(1 - r^2)^2, and the triangles Bn1CnBnB_{n-1}C_nB_n have disjoint interiors.

The union's area is the geometric series 90r2(1r2)1(1r2)2=90(1r2)2r2=90336/625961/625=90336961.\frac{90\,r^2(1 - r^2)}{1 - (1 - r^2)^2} = \frac{90(1 - r^2)}{2 - r^2} = 90 \cdot \frac{336/625}{961/625} = \frac{90 \cdot 336}{961}. Since 961=312961 = 31^2 shares no factor with 90336=30240,90 \cdot 336 = 30240, the answer is q=961.q = 961.

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