Giải thích các bước giải:
Ta có:
$a.AC//BD\rightarrow \widehat{CAO}=\widehat{OBD}\rightarrow \widehat{ACO}=\widehat{ODB}$
$\rightarrow \Delta OAC=\Delta OBD(g.c.g) \rightarrow AC=BD\rightarrow đpcm$
$b.\rightarrow \widehat{AOC}=\widehat{BOD}\rightarrow \widehat{AOC}+\widehat{COB}=\widehat{BOD} +\widehat{COB}$
$\rightarrow 180^o=\widehat{COD}\rightarrow \text{C,O,D thẳng hàng}\rightarrow đpcm$