The incircle of the triangle ABC touches BC, CA, AB at D, E, F respectively. AD meets the circle again at Q. Show that the line EQ passes through the midpoint of AF iff AC=BC.
Solution
Solution:
∠AQM=∠EQD (opposite angle) =∠EDC (CD tangent to circle EQD) =(180∘−∠C)/2=∠A/2+∠B/2(∗).
MF2=MQ⋅ME (MF tangent to circle FQE). So AM=AF is equivalent to AM2=MQ⋅ME or AM/MQ=ME/AM. But since triangles AMQ and EMA have a common angle M, AM/MQ=ME/AM iff they are similar, and hence iff ∠AQM=∠A. Using (∗), AM=AF iff ∠A=∠B.
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Source: MathNet,
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