$\displaystyle \sum_{i=1}^n Var(Y_i-\hat{Y}_i)+[E(Y_i-\hat{Y}_i)]^2$=
To be more specified: This is the intermediate step in proving $\displaystyle E(MSE)=\sigma$ in regression study. furthermore, $\displaystyle \hat{Y}_i=b_0+b_1X_i$