Nếu hình như hình 1:
Kẻ $OC//a$
$→OC//a//b$
$→\widehat{A}=\widehat{O_1}=30^{\circ}$ (so le trong)
$→\widehat{B}=\widehat{O_2}=40^{\circ}$ (so le trong)
$→\widehat{AOB}=\widehat{O_1}+\widehat{O_2}=30^{\circ}+40^{\circ}=70^{\circ}$
Nếu đề như hình 2:
Kẻ $OC//a$
$→OC//a//b$
$→\widehat{A}+\widehat{O_1}=180^{\circ}$ mà $\widehat{A}=30^{\circ}→\widehat{O_1}=150^{\circ}$
$→\widehat{B}+\widehat{O_2}=180^{\circ}$ mà $\widehat{B}=40^{\circ}→\widehat{O_2}=140^{\circ}$
$→\widehat{AOB}=\widehat{O_1}+\widehat{O_2}=150^{\circ}+140^{\circ}=290^{\circ}$
