Maths Olympiad Prep

Library / /4 of 48

Geometry Difficulty 6.7 National olympiad Find the answer

Let A1,A2,A3A_{1}, A_{2}, A_{3} be three points in the plane, and for convenience, let A4=A1,A5=A2A_{4}=A_{1}, A_{5}=A_{2}. For n=1,2n=1,2, and 3, suppose that BnB_{n} is the midpoint of AnAn+1A_{n} A_{n+1}, and suppose that CnC_{n} is the midpoint of AnBnA_{n} B_{n}. Suppose that AnCn+1A_{n} C_{n+1} and BnAn+2B_{n} A_{n+2} meet at DnD_{n}, and that AnBn+1A_{n} B_{n+1} and CnAn+2C_{n} A_{n+2} meet at EnE_{n}. Calculate the ratio of the area of triangle D1D2D3D_{1} D_{2} D_{3} to the area of triangle E1E2E3E_{1} E_{2} E_{3}.

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

Solution

Let GG be the centroid of triangle ABCA B C, and also the intersection point of A1B2,A2B3A_{1} B_{2}, A_{2} B_{3}, and A3B1A_{3} B_{1}. By Menelao's theorem on triangle B1A2A3B_{1} A_{2} A_{3} and line A1D1C2A_{1} D_{1} C_{2}, A1B1A1A2D1A3D1B1C2A2C2A3=1D1A3D1B1=23=6D1B1A3B1=17\frac{A_{1} B_{1}}{A_{1} A_{2}} \cdot \frac{D_{1} A_{3}}{D_{1} B_{1}} \cdot \frac{C_{2} A_{2}}{C_{2} A_{3}}=1 \Longleftrightarrow \frac{D_{1} A_{3}}{D_{1} B_{1}}=2 \cdot 3=6 \Longleftrightarrow \frac{D_{1} B_{1}}{A_{3} B_{1}}=\frac{1}{7} Since A3G=23A3B1A_{3} G=\frac{2}{3} A_{3} B_{1}, if A3B1=21tA_{3} B_{1}=21 t then GA3=14t,D1B1=21t7=3t,A3D1=18tG A_{3}=14 t, D_{1} B_{1}=\frac{21 t}{7}=3 t, A_{3} D_{1}=18 t, and GD1=A3D1A3G=18t14t=4tG D_{1}=A_{3} D_{1}-A_{3} G=18 t-14 t=4 t, and GD1GA3=414=27\frac{G D_{1}}{G A_{3}}=\frac{4}{14}=\frac{2}{7} Similar results hold for the other medians, therefore D1D2D3D_{1} D_{2} D_{3} and A1A2A3A_{1} A_{2} A_{3} are homothetic with center GG and ratio 27-\frac{2}{7}. By Menelao's theorem on triangle A1A2B2A_{1} A_{2} B_{2} and line C1E1A3C_{1} E_{1} A_{3}, C1A1C1A2E1B2E1A1A3A2A3B2=1E1B2E1A1=312=32A1E1A1B2=25\frac{C_{1} A_{1}}{C_{1} A_{2}} \cdot \frac{E_{1} B_{2}}{E_{1} A_{1}} \cdot \frac{A_{3} A_{2}}{A_{3} B_{2}}=1 \Longleftrightarrow \frac{E_{1} B_{2}}{E_{1} A_{1}}=3 \cdot \frac{1}{2}=\frac{3}{2} \Longleftrightarrow \frac{A_{1} E_{1}}{A_{1} B_{2}}=\frac{2}{5} If A1B2=15uA_{1} B_{2}=15 u, then A1G=2315u=10uA_{1} G=\frac{2}{3} \cdot 15 u=10 u and GE1=A1GA1E1=10u2515u=4uG E_{1}=A_{1} G-A_{1} E_{1}=10 u-\frac{2}{5} \cdot 15 u=4 u, and GE1GA1=410=25\frac{G E_{1}}{G A_{1}}=\frac{4}{10}=\frac{2}{5} Similar results hold for the other medians, therefore E1E2E3E_{1} E_{2} E_{3} and A1A2A3A_{1} A_{2} A_{3} are homothetic with center GG and ratio 25\frac{2}{5}. Then D1D2D3D_{1} D_{2} D_{3} and E1E2E3E_{1} E_{2} E_{3} are homothetic with center GG and ratio 27:25=57-\frac{2}{7}: \frac{2}{5}=-\frac{5}{7}, and the ratio of their area is (57)2=2549\left(\frac{5}{7}\right)^{2}=\frac{25}{49}.

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.