a) Có: Ax // Oz (gt)
Ax // By (gt)
⇒ Oz // By (tính chất 3 đường thẳng song song)
b) Có: Oz // Ax (gt)
\(\Rightarrow\widehat{OAx}=\widehat{AOz}\) (2 góc so le trong)
Mà \(\widehat{OAx}=35^o\)
\(\Rightarrow\widehat{AOz}=35^o\)
Có: Oz // By (cmt)
\(\Rightarrow\widehat{zOB}+\widehat{OBy}=180^o\) (2 góc trong cùng phía)
Thay số: \(\widehat{zOB}+140^o=180^o\)
\(\Rightarrow\widehat{zOB}=40^o\)
Mặt khác: \(\widehat{AOB}=\widehat{AOz}+\widehat{zOB}\)
Thay số: \(\widehat{AOB}=35^o+40^o\)
\(\Rightarrow\widehat{AOB}=75^o\)
