a) Vì \(\widehat {BAC} = {90^0}\) (gt) nên \(\widehat {{A_1}} + \widehat {{A_2}} = {90^0}\)

mà \(\widehat {{C_1}} + \widehat {{C_2}} = {90^0} \Rightarrow \widehat {{A_1}} = \widehat {{C_1}}\) hay \(\widehat {DAB} = \widehat {ACE}\)

b) Vì \(\widehat {{A_1}} = \widehat {{C_1}}\) (cmt), AB = AC (gt), \(\widehat {ADB} = \widehat {CEA} = {90^0}\)

Nên \(\Delta ABD = \Delta CAE\) (cạnh huyền - góc nhọn)

c) Vì \(\Delta ABD = \Delta CAE \Rightarrow BD = AE,CE = AD\)

nên BD + CE = EA + AD = DE