We have,
x=0,y=0,x=1,y=1
Now,
Let
O(x1,y1)=(0,0)
A(x2,y2)=(1,0)
B(x3,y3)=(1,1)
C(x3,y3)=(0,1)
Are the vertices of square.
Now,
Equation of the straight line passing through the points (x1,y1) and (x2,y2) is,
y−y1=y2−y1x2−x1(x−x1)
Now, equation of diagonal OB is
y−0=1−01−0(x−0)
⇒y=x
Now, equation of diagonal AC is
y−0=1−00−1(x−1)
⇒y=−(x−1)
⇒y=−x+1
⇒x+y+1=0
Hence, this is the answer.