协方差公式推导 cov(X,Y)=∑ni=1(Xi?X?)(Yi?Y?)n=E[(X?E[X])(Y?E[Y])] cov(X,Y)=∑i=1n(Xi?X?)(Yi?Y?)n=E[(X?E[X])(Y?E[Y])]
=E[XY?E[X]Y?XE[Y]+E[X]E[Y]] =E[XY?E[X]Y?XE[Y]+E[X]E[Y]]
因为均值计算是线性的,即(a和b均为常数): E[aX+bY]=aE[X]+bE[Y] E[aX+bY]=aE[X]+bE[Y]
则我们有: E[XY?E[X]Y?XE[Y]+E[X]E[Y]] E[XY?E[X]Y?XE[Y]+E[X]E[Y]]
=E[XY]?E[X]E[Y]?E[X]E[Y]+E[X]E[Y] =E[XY]?E[X]E[Y]?E[X]E[Y]+E[X]E[Y]
=E[XY]?E[X]E[Y]