方差存在的條件:方差不存在是什麼意思[朗讀]

∵d(x+y)=e(x+y)2-[e(x+y)]2=e[x+y-e(x)-e(y)]2=e[x-e(x)]2+e[y-e(y)]2+2e[x-e(x)][y-e(y)]=d(x)+d(y)+2cov(x,y)=d(x)+d(y)+2rxyd(x)?d(y),∴由題設x和y的方差存在且不等於0:d(x)≠0,d(y)≠0,得:d(x+y)=dx+dy?rxy=0,故選:c。