\(\widehat{AOD}=180^0-\widehat{DOB}\)
\(\widehat{BOC}=180^0-\widehat{AOC}\)
Mà \(\widehat{DOB}=\widehat{AOC}\) (Gt)
=>\(\widehat{AOD}=\widehat{COB}\)
Xét \(\Delta AOD\) và \(\Delta COB\) có :
AO = CO (Gt)
\(\widehat{AOD}=\widehat{COB}\)
DO = BO (Gt)
=>\(\Delta AOD\) = \(\Delta COB\) (c.g.c)
=> AD = BC (2 cạnh tương ứng)