Ta có: $\widehat{zBm}+\widehat{mBt}=180^{\circ}$ (hai góc kề bù)
suy ra $60^{\circ}+\widehat{mBt}=180^{\circ}$ suy ra$\widehat{mBt}=180^{\circ}-60^{\circ}=120^{\circ}$
Ta có: $\widehat{yAm}=\widehat{tBm}(=120^{\circ})$ mà $\widehat{yAm}$ và $\widehat{tBm}$ là hai góc đồng vị.
Do đó xy//zt