Maths Olympiad Prep

Library / Baltic Way competition problems /10 of 45

Baltic Way competition problems

Geometry Difficulty 4.7 AIME Prove it Baltic Way

Problem:
Let ABCABC be a triangle such that ∣BC∣∣AB∣−∣BC∣=∣AB∣+∣BC∣∣AC∣\frac{|BC|}{|AB| - |BC|} = \frac{|AB| + |BC|}{|AC|}. Determine the ratio ∠A:∠C\angle A : \angle C.

Solution

Solution:
Answer: 1:21 : 2.

Denote ∣BC∣=a|BC| = a, ∣AC∣=b|AC| = b, ∣AB∣=c|AB| = c. The condition ac−a=c+ab\frac{a}{c - a} = \frac{c + a}{b} implies c2=a2+abc^2 = a^2 + ab and
ca+b=ac. \frac{c}{a + b} = \frac{a}{c}.
Let DD be a point on ABAB such that ∣BD∣=aa+b⋅c|BD| = \frac{a}{a + b} \cdot c (see Figure 5). Then
∣BD∣∣BC∣=ca+b=ac=∣BC∣∣BA∣ \frac{|BD|}{|BC|} = \frac{c}{a + b} = \frac{a}{c} = \frac{|BC|}{|BA|}
so triangles BCDBCD and BACBAC are similar, implying ∠BCD=∠BAC\angle BCD = \angle BAC. Also, ∣AC∣∣BC∣=∣AD∣∣BD∣\frac{|AC|}{|BC|} = \frac{|AD|}{|BD|} yields ∣BC∣∣BD∣=∣AC∣∣AD∣\frac{|BC|}{|BD|} = \frac{|AC|}{|AD|}, and hence by the bisector theorem CDCD is the bisector of ∠BCA\angle BCA. So the ratio asked for is 1:21 : 2.

Figure 1
Figure 5

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.