a/ $\widehat{xEt}+\widehat{DEt}=180^\circ$
$→\widehat{DEt}=180^\circ-35^\circ=145^\circ$
$→\widehat{DEt}=\widehat{EDm}=145^\circ$
mà 2 góc ở vị trí so le trong
$→Et//Dm$
b/$Dm//Fy$
$→\widehat{FDm}=\widehat{DFy}=123^\circ$
mà $\widehat{EDm}+\widehat{FDm}+\widehat{EDF}=360^\circ$
$→\widehat{EDF}=360^\circ-145^\circ-123^\circ=92^\circ$
c/ $\begin{cases}Dm//Fy\\Et//Dm\end{cases}→Et//Fy$
