lericjee 发表于 2010-6-7 23:40 
这个怎么就等于0了呢,我查了几本书,都是说显然- -我百思不得什解
拜托解答·
It is the same as
sum of (e(i)*x(i)) =0
It is a direct result of FIRST ORDER CONDITION.
This is coresponding to the regression assumption of E( u*x)=0 or the regressor is uncorrelated with error term.