设X是1~80中任取的1个数字，则 P{X = K} = 1/80, K = 1,2,...,80.EX = [1 + 2 + ... + 80](1/80) = 80*81/[2*80] = 81/2.EX^2 = [1^2 + 2^2 + ... + 80^2](1/80) = 80*81*161/[6*80] = 27*161/2DX = EX^2 - (EX)^2 = 27*161/2 - (81/2)^2 = 2133/4设Y = [从1～80任意取20个数字的和]/20则，EY = EX = 81/2,DY = DX/20 = 2133/80.Y近似服从均值= 81/2,方差=2133...
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