ABCD is a square, the length of whose side is a. Taking AB and AD as the coordinate axes, the equation of the
circle passing through the vertices of the square, is
According to the figure A(0, 0), B(a, 0) and D(0, a)
and centre is (a2,a2).
Therefore, the equation of circle
(x−a2)2+(y−a2)2=a22
⇒x2+y2−ax−ay=0.