3) Qua O kẻ đường thẳng $//Ax$;By là Cz
Vì:$Cz//Ax$(theo hình )
$=>\widehat{xAO}=\widehat{AOz}$($=50^o$)(2 góc SLT=nhau)(1)
Vì:$Cz//By$(theo hình)
$=>\widehat{zOB}+\widehat{OBy}=180^o$
hay $\widehat{zOB}+140^o=180^o$
$=>\widehat{zOB}=180^o- 140^o=40^o$
Có :$\widehat{AOB}=\widehat{AOz}+\widehat{zOB}$
Hay$\widehat{ AOB}=50^o+40^o$
$=>\widehat{AOB}=90^o$