1. The problem statement, all variables and given/known data If the line ## hk + ky = 1 ## touches the circle ## x^2 + y^2 = a^2 ##, then locus of the points (h,k) is a circle of radius : a h k 1/a 2. Relevant equations NA 3. The attempt at a solution Tried differentiating circle equation (Is differentiation of use here?, also don't know if it is a calculus or pre-calculus problem. ) Got y' = -x/y Differentiated line equation and thinking h,k are constants which is not true I got y' = 0 I have to substitute something but what?