$\Delta AED \sim \Delta BAD (g-g) \Rightarrow \frac{AB}{AE}=\frac{BD}{AD}$=> $\frac{AC}{AE}=\frac{BD}{CD}$
=> $\frac{AC}{BD}=\frac{AE}{CD}$
=> $\Delta ACE \sim \Delta CBD (c-g-c)$
=> $\widehat{CBD}=\widehat{ACE}$
Mà $\widehat{ABC}=\widehat{ACB} \Rightarrow \widehat{ABD}+\widehat{CBD}=\widehat{ACE}+\widehat{ECB}$
=> $\widehat{ABD}=\widehat{ECB}$
=> $\widehat{DAE}=\widehat{ECB}$