1.Exception: if the new regressor is perfectly multicollinear with the original regressors , then OLS cannot be implemented.
例外：如果这个新解释变量与原有的解释变量完全共线，那么OLS不能使用。
2.This algebraic fact follows because the sum of squared residuals never increase when additional regressor are added to the model.
此代数事实成立，因为当模型加入更多回归元时，残差平方和绝不会增加。