In Section 9.3 we emphasized the notion of most efficient

In Section 9.3 we emphasized the notion of "most efficient estimator" by comparing the variance of two unbiased estimators and  however this does not take into account bias in case one or both estimators are not unbiased. Consider the quantity where MSE denotes mean squared error. The MSE is often used to compare two estimators  and  of 6 when either or both is unbiased because (i) it is intuitively reasonable and (ii) it accounts for bias. Show that MSE can be written