MathNet 2016-part-01 — 184 problems (EN–VI) Question 128

Let $A$, $B$, $C$ denote the intersection points of the diagonals $A_1A_4$ and $A_2A_5$, $A_1A_6$ and $A_2A_7$, $A_1A_9$ and $A_2A_{10}$ of the regular decagon $A_1A_2...A_9A_{10}$, respectively.
Find the angles of the triangle $ABC$.
(Folklore)