If O is the origin and OP, OQ are the tangents to the circles x²+y²+2gx+2fy+c=0 then the circumcentre of the ΔOPQ is

If a circle passes through the point (a, b) and cuts the circle x²+y²=k² orthogonally, the equation of the locus of its centre is

Let A be the centre of the circle x²+y²-2x-4y-20=0. Suppose that the tangent at the point B(1, 7) and D(4, -2) on the circle meet at the point C. The area of the quadrilateral ABCD is