MathNet 2016-part-05 — 184 problems (EN–VI) Question 43

Let $D$ be the point in the interior of a triangle $ABC$ such that $AB = ab$, $AC = ac$, $AD = ad$, $BC = bc$, $BD = bd$ and $CD = cd$. Prove that $\angle ABD + \angle ACD = 60^\circ$.