11.设x1,···,x0是来自N (μ1,σ^2)的样本,y1,···,ym是来自N(μ2,σ^2)的样本,c,d是任-|||-意两个不为0的常数,证明:-|||-=dfrac (c(overline {x)-(mu )_(1))+d(overline (y)-(mu )_(2))}(ssqrt {dfrac {{e)^2}(n)+dfrac ({d)^2}(m)}}-t(n+m-2),-|||-其中 ({s)_(x)}^2=dfrac ((n-1){{s)_(x)}^2+(m-1)({s)_(y)}^2}(n+m-2)