The equations of the diagonals of square formed by lines x =0 , y = 0, x = 1  and y = 1 are

The coordinates of the vertices of the square are A(0, 0), B(0, 1), C(1, 1), and D(1, 0). Now, the equation of AC is y = x and that of BD isy−1=−11(x−0) or  x+y=1