Qua điểm $O$ , kẻ đoạn thẳng $CD // Ax // By$
Ta có :
$\widehat{yBO}+\widehat{BOC}=180°$ ( Hai góc trong cùng phía )
$⇔130°+\widehat{BOC}=180°$
$⇔\widehat{BOC}=50°$
$\widehat{xAO}+\widehat{AOD}=180°$ ( Hai góc trong cùng phía )
$⇔115°+\widehat{AOD}=180°$
$⇔\widehat{AOD}=65°$
Mà $\widehat{BOC}+\widehat{AOB}+\widehat{AOD}=180°$
$⇔\widehat{AOB}+65°+50°=180°$
$⇔\widehat{AOB}+115°=180°$
$⇔\widehat{AOB}=65°$
Vậy $\widehat{AOB}=65°$
