一条统计与概率的英文题

一条统计与概率的英文题Suppose that we are to observe two independent random samples:Y1,Y2,...,Yn denoting a random sample from a normal distribution with mean µ1 and variance σ21;(2上1下） and X1,X2,...,Xm denoting a random sample from another normal distribution with mean µ2 and variance σ22（2上2下）An approximation for µ1− µ2 is given by Y − X,the difference between the sample means.Find _ _E [Y − X]\2 _ _and Var\3(Y − X)\4.