$$\eqalign{
& Ap\,\,dung\,\,tinh\,\,chat\,\,duong\,\,phan\,\,giac\,\,ta\,\,co: \cr
& {{AD} \over {CD}} = {{AB} \over {BC}} \cr
& {{AE} \over {BE}} = {{AC} \over {BC}} \cr
& Ma\,\,AB = AC \Rightarrow {{AD} \over {CD}} = {{AE} \over {BE}} \cr
& \Rightarrow ED//BC\,\,\left( {DL\,\,Ta - let\,\,dao} \right) \cr
& \Rightarrow BEDC\,\,la\,\,hinh\,\,thang. \cr
& Lai\,\,co:\,\,\widehat {ABC} = \widehat {ACB}\,\,\left( {gt} \right) \cr
& \Rightarrow BEDC\,\,la\,\,hinh\,\,thang\,\,can. \cr
& Ta\,\,co:\,\,DE//BC \cr
& \Rightarrow \widehat {EDB} = \widehat {DBC}\,\,\left( {so\,\,le\,\,trong} \right) \cr
& Ma\,\,\widehat {DBC} = \widehat {ABD}\,\,\left( {gt} \right) \cr
& \Rightarrow \widehat {EDB} = \widehat {ABD} \Rightarrow \Delta BDE\,\,can\,\,tai\,\,E. \cr
& \Rightarrow ED = EB\,\,\left( {dpcm} \right) \cr} $$
