a) ta co : ΔACH ∽ ΔCED (c-g-c)
=> goc CDE = goc CAH 
xet Δ CED va ΔABH , co:
AB = DC = 1/2AC 
goc CDE = goc CAH (cmt)
=> ΔCED = ΔABH (canh huyen - goc nhon)
b) tu cau a => CE = EH
va DE canh chung
=> ΔDHE = ΔDCE ( hai canh goc vuong)
=> DH = DC 
=> ΔDHC can tai D