The K.E. of pendulum =K.E.=21K(a2−y2) and P.E. of pendulum =21KY2 when y=0 (mean position) K.E. is maximum and PE is 0 . When y=0, K.E. is 0 and P.E. is maximum. ∴ The displacement between position of maximum potential energy and maximum kinetic energy is ±a.