MathDB
Sequence of triangles

Source: South Africa 1997

October 8, 2005
geometrylimit

Problem Statement

From an initial triangle ΔA0B0C0\Delta A_0B_0C_0, a sequence of triangles ΔA1B1C1\Delta A_1B_1C_1, A2B2C2A_2B_2C_2, ... is formed such that, at each stage, Ak+1A_{k + 1}, Bk+1B_{k + 1} and Ck+1C_{k + 1} are the points where the incircle of ΔAkBkCk\Delta A_kB_kC_k touches the sides BkCkB_kC_k, CkAkC_kA_k and AkBkA_kB_k respectively. (a) Express Ak+1Bk+1Ck+1\angle A_{k + 1}B_{k + 1}C_{k + 1} in terms of AkBkCk\angle A_kB_kC_k. (b) Deduce that, as kk increases, AkBkCk\angle A_kB_kC_k tends to 6060^{\circ}.