dylanzju The elements of statistics 2nd version. Exercise 2.7 <bblatex>E_{Y|X}=(f(x_0)-\hat{f(x_0)})^2</bblatex> 和 <bblatex>E_{X,Y}=(f(x_0)-\hat{f(x_0)})^2</bblatex>分解成Bias和Var两个部分~
