Question

Foucault's pendulum consists of a long light inextensible string of length a and a heavy bob of mass m. The pendulum is attached to a point fixed relative to the surface of the Earth. Take the origin O to be the equilibrium position of the bob with Oxyz parallel unit vectors to i, j, k. If the position vector of the bob is r=x+yj+zk show that a^2=|r-ak|^2