$\widehat{P_2}=\widehat{P_4}=130^o$ (đối đỉnh)
$\widehat{P_1}=180^o-\widehat{P_2}=50^o=\widehat{P_3}$ (đối đỉnh)
Do $m//n$ nên $\widehat{P_2}=\widehat{Q_1}=130^o$ (đồng vị)
$\widehat{Q_1}=\widehat{Q_3}=130^o$ (so le trong)
$\widehat{P_1}=\widehat{Q_4}=50^o$ (đồng vị)
$\widehat{Q_4}=\widehat{Q_2}=50^o$ (so le trong)