If the chord of contact of tangents drawn from the point (h, k) to the circle x2+y2=a2  subtends a right angle at the centre, then

The equation of the chord of contact of tangents drawn from the point (h, k) to the circle x2+y2=a2 is hx+ky=a2 The combined equation of OQ and OR is x2+y2=a2hx+kya22Since OQ is perpendicular to OR. Therefore,  Coeff. of x2+ Coeff. of y2=0⇒2a2=h2+k2