,均值,方差,抽样分布方差 随机变量x,E(x),Q(x)²,Q(x)²/n 随机变量y,E(y),Q(y)²,Q(y)²/n 随机变量z=x+y,E(x)+E(y),Q(x)²+Q(y)²,Q(x)²/n+Q(y)²/n 随机变量z=x-y,E(x)-E(y),Q(x)²+Q(y)²,Q(x)²/n+Q(y)²/n