Let the vertices be A(0,0); B(5,5) and C(-5,5).
Remember that in a right triangle Pythagoras theorem will be followed.
Also for an isosceles triangle two sides must be of the same length.
By distance formula:
The triangle is clearly isosceles as AB = AC.
AB2 + AC2 = 50 + 50 = 100 = 102 = BC2