a, Ox//AB
=>\(\widehat{BOx}=\widehat{ABO}=30^o\) (so le trong)
b, +)\(\widehat{BOD}=\widehat{BOx}+\widehat{DOx}=110^o\)
\(=>\widehat{Dox}=\widehat{BOD}-\widehat{BOx}\)\(=110^o-30^o=80^o\)
+)\(\widehat{DOx}=\widehat{ODC}=80^o\) mà 2 góc này nằm ở vị trí so le trong
=>Ox//CD
+)Ox//CD
Ox//AB
=>AB//CD