Maths Olympiad Prep

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Geometry Difficulty 4.9 AIME Prove it Ibero-American Mathematical Olympiad

Problem:

The incircle of the triangle ABCABC touches BCBC, CACA, ABAB at DD, EE, FF respectively. ADAD meets the circle again at QQ. Show that the line EQEQ passes through the midpoint of AFAF iff AC=BCAC = BC.

Solution

Solution:

Figure 1

AQM=EQD\angle AQ M = \angle EQ D (opposite angle) =EDC= \angle EDC (CDCD tangent to circle EQDEQD) =(180C)/2=A/2+B/2= \left(180^\circ - \angle C\right) / 2 = \angle A / 2 + \angle B / 2 ()(*).

MF2=MQMEMF^2 = MQ \cdot ME (MFMF tangent to circle FQEFQE). So AM=AFAM = AF is equivalent to AM2=MQMEAM^2 = MQ \cdot ME or AM/MQ=ME/AMAM / MQ = ME / AM. But since triangles AMQAMQ and EMAEMA have a common angle MM, AM/MQ=ME/AMAM / MQ = ME / AM iff they are similar, and hence iff AQM=A\angle AQ M = \angle A. Using ()(*), AM=AFAM = AF iff A=B\angle A = \angle B.

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