Giải thích các bước giải:
Ta có :
$\widehat{O'AC}=\widehat{O'CA},\widehat{OAE}=\widehat{OEA}\rightarrow \widehat{O'CA}=\widehat{OEA}$
Mà $\widehat{DOE}=\widehat{BO'C}(=2\widehat{BAE})\rightarrow \widehat{DEO}=\widehat{O'CB}$
$\rightarrow\widehat{DEA}=\widehat{DEO}+\widehat{OEA}=\widehat{BCO'}+\widehat{O'CA}=\widehat{BCA}$
$\rightarrow BC// DE$