MathNet 2026 — Part 08 Question 73

Squares $A B D E$, $B C F G$ and $C A H I$ are drawn exterior to a triangle $A B C$. Parallelograms $D B G X$, $F C I Y$ and $H A E Z$ are completed. Prove that $\angle A Y B + \angle B Z C + \angle C X A = 90^{\circ}$.