Let the incircle of triangle ABC be tangent to sides AB, AC, and AB at D, E, and F, respectively. Prove that triangle DEF is acute.
Pick one of the following solutions:
https://web2.0calc.com/questions/geometry_16414
https://web2.0calc.com/questions/let-the-incircle-of-triangle-abc-be-tangent-to-sides
https://web2.0calc.com/questions/sorry-cphill-here-is-the-full-geometry-question
+194 
