The formula of kinetic energy in S.H.M. is, K.E.=21mω2(A2−x2)2, where A is the amplitude of the S.H.M. the formula of potential energy is, P.E.=21mω2x2, so the net energy is defined as, T.E.=P.E.+K.E.=21mω2A2 so the total energy is always constant. Since, energy changes from potential to kinetic and vice-versa the average K.E. in one time period is equal to the average P.E. in one time period.