In the acute triangle ABC, the point I is the center of the inscribed the circle, the point O is the center of the circumscribed circle and the point Ia is the center the excircle tangent to the side BC and the extensions of the sides AB and AC. Point A′ is symmetric to vertex A with respect to the line BC. Prove that ∠IOIa=∠IA′Ia. geometryincentercircumcircleequal angles