Maths Olympiad Prep

Library / /1 of 64

Geometry Difficulty 7.4 National olympiad, round 2 Find the answer

For i=1,2i = 1,2 let TiT_i be a triangle with side lengths ai,bi,cia_i, b_i, c_i, and area AiA_i. Suppose that a1a2,b1b2,c1c2a_1 \le a_2, b_1 \le b_2, c_1 \le c_2, and that T2T_2 is an acute triangle. Does it follow that A1A2A_1 \le A_2?

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

Solution

Yes, it does follow. For i=1,2i=1,2, let Pi,Qi,RiP_i, Q_i, R_i be the vertices of TiT_i opposite the sides of length ai,bi,cia_i, b_i, c_i, respectively. We first check the case where a1=a2a_1 = a_2 (or b1=b2b_1 = b_2 or c1=c2c_1 = c_2, by the same argument after relabeling). Imagine T2T_2 as being drawn with the base Q2R2Q_2R_2 horizontal and the point P2P_2 above the line Q2R2Q_2R_2. We may then position T1T_1 so that Q1=Q2Q_1 = Q_2, R1=R2R_1 = R_2, and P1P_1 lies above the line Q1R1=Q2R2Q_1R_1 = Q_2R_2. Then P1P_1 also lies inside the region bounded by the circles through P2P_2 centered at Q2Q_2 and R2R_2. Since Q2\angle Q_2 and R2\angle R_2 are acute, the part of this region above the line Q2R2Q_2R_2 lies within T2T_2. In particular, the distance from P1P_1 to the line Q2R2Q_2R_2 is less than or equal to the distance from P2P_2 to the line Q2R2Q_2R_2; hence A1A2A_1 \leq A_2.

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.