Probability density function(概率密度函数) fx(x)= dFx(x) dx Moment(矩)： 1st moment Mean value(Mathematical expectation(数学期望) E(X)=xP(x)dx=ux 2nd moment E(X2)=[x2P(x)dxl Probability density function(概率密度函数) l Moment（矩）: 1st moment Mean value (Mathematical expectation(数学期望) 2nd moment dx dF x f x X X ( ) ( )     d  X E(X ) xP(x) x    E(X )  x P(x)dx 2 2