rubber<-data.frame(

  X1=c(65,70,70,69,66,67,68,72,66,68),

  X2=c(45,45,48,46,50,46,47,43,47,48),

  X3=c(27.6,30.7,31.8,32.6,31.0,31.3,37.0,33.6,33.1,34.2)

);rubber

colMeans(rubber)#按列求均值

cov(rubber)#求协方差矩阵

如何求E(X1,X2|X3),和cov(X1,X2|X3)?