Show that, with a non-uniform quantizer, the mean-square value

Show that, with a non-uniform quantizer, the mean-square value

Show that, with a non-uniform quantizer, the mean-square value of the quantization error is approximately equal to (1/12) Si ?2i pi, where ?i is the ith step size and pi is the probability that the input signal amplitude lies within the ith interval. Assume that the step size ?i is small compared with the excursion of the input signal.