Dear Student,
Consider ABC as an isosceles triangle, with AB = AC;
(1/2)AB = (1/2)AC ----- (1)
If D is the midpoint of AB, then AD = (1/2)AB --------- (2)
Similarly if E is the midpoint of AC, then AE = (1/2)AC ------ (3)
Hence, from (1), (2) & (3), AD = AE
As two sides of the triangle ADE are equal, the triangle ADE is isosceles.
Regards
Hope it Helps :)