Question 5
Outside regular nonagon $ABCDEFGHI$, triangles $XAB,YBC,ZCD,TDE$ are constructed. Their angles $AXB,BYC,CZD,DTE$ all equal $20^\circ$, and $\angle XAB+60^\circ=\angle YBC+40^\circ=\angle ZCD+20^\circ=\angle TDE$. Prove that $X,Y,Z,T$ are concyclic.
