The centre C of a variable circle passing througha fixed point (a, 0), a > 0, touches the line y = x.Locus of C is a parabola whose directrix is
x + y = 0
x+y=a
x−y=0
x−y=a
C is equidistant from the point (a, 0) and the line
y=x, so locus of C is a parabola with directrix
y = x i.e. x – y = 0.