a) xét \(\Delta OAD\)và \(\Delta BOC\)có :
\(OA=OB\left(gt\right)\)
\(O\)Chung
\(OD=OC\left(gt\right)\)
\(\Rightarrow\Delta OAD=\Delta OBC\)
b) \(AC-OC-OA=OD-OB=BD\)
Xét \(\Delta ADC\)và \(\Delta BCD\)
\(CD\)Chung.
\(AC=BD\)
\(AD=BC\left(\Delta OAD=\Delta OBC\right)\)
\(\Rightarrow\Delta ADC=\Delta BCD\)
\(\Rightarrow\widehat{CAD}=\widehat{CBD}\)