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Bulgaria

Bulgaria geometry

Problem

Let be the incircle of a . A line, parallel to touches and intersects the sides and at points and . Define the points and in a similar way. Prove that
Solution
Let and be the common points of with the sides and , respectively, and let , and . Since , then and hence and . Analogously, , , and . Then the given inequality is equivalent to i.e.

The last inequality follows by the well-known inequalities

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTangentsHomothetyTriangle inequalitiesCauchy-Schwarz