Giải thích các bước giải:
a.Trong $\widehat{AOC}$ kẻ tia $OE//AB$
$\to\widehat{AOE}+\widehat{OAB}=180^o$(trong cùng phía)
$\to \widehat{AOE}=180^o-\widehat{OAB}=45^o$
$\to\widehat{EOC}=\widehat{AOC}-\widehat{AOE}=75^o$
$\to \widehat{EOC}+\widehat{OCD}=75^o+105^o=180^o$
$\to OE//CD$
Mà $AB//OE$
$\to AB//CD$
b.Trong $\widehat{AOC}$ kẻ tia $OE//AB$
$\to \widehat{AOE}=\widehat{OAB}=75^o$(So le trong)
$\to \widehat{EOC}=\widehat{AOC}-\widehat{AOE}=30^o$
$\to \widehat{EOC}=\widehat{OCD}$
$\to OE//CD$
