$\text{--quachtantruong--}$

$\text{Kẻ tia Bz // Ax}$

$\text{⇒ $\widehat{xAB}$ = $\widehat{ABz}$ (2 góc so le trong)}$

$\text{$\widehat{ABz}$ + $\widehat{zBC}$ + $\widehat{ABC}$ = $360^\circ$}$

$\text{Mà $\widehat{xAB}$ + $\widehat{BCy}$ + $\widehat{ABC}$ = $360^\circ$}$

$\text{$\widehat{ABz}$ + $\widehat{zBC}$ + $\widehat{ABC}$ = $\widehat{xAB}$ + $\widehat{BCy}$ + $\widehat{ABC}$}$

$\text{⇒ $\widehat{ABz}$ + $\widehat{zBC}$= $\widehat{xAB}$ + $\widehat{BCy}$}$

$\text{Mà $\widehat{xAB}$ = $\widehat{ABz}$}$

$\text{nên $\widehat{zBC}$= $\widehat{BCy}$}$

$\text{Mà 2 góc này ở vị trí so le trong}$

$\text{nên Bz // Cy}$

$\text{Mà Ax // Bz}$

$\text{⇒ Ax // Cy}$

$\text{Chúc bạn học tốt!!}$