In ΔABD and ΔAEC
AB=AC (given) …(i)
∠ABC=∠ACB …(ii) (angle opposite to equal sides are equal)
Also BD=EC (given) …(iii)
From (i), (ii) and (iii), we have
ΔABD=ΔAEC (by SAS congruency property)
AD=AE (by c.p.c.t)
Hence, ΔADE is an isosceles triangle.
Hence proved.