OLS estimator proof.统计学,经济计量学,证明.

OLS estimator proof.统计学,经济计量学,证明.
Prove that sample mean x-bar is an OLS estimator of mu.You have only one variable X,and data Xi,i=1,2,.n.You have to minimize an ESS,but it is not the one associated with the 2-variable regression model.It is simpler.
教授的原题目如下,

注：对mu 求一阶条件