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*161/2
DX = 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=2133/80的正态分布。
记 d = (2133/80)^(1/2) = 5.1635743434175516865242169119168
G(u)为标准正态分布的分布函数。
[G(-|v|) = 1 - G(|v|), G(|v|)的值可以查正态分布的分布函数表]
则,
P{a < 从1~80任意取20个数字的和 < b}
= P{a/20 < Y < b/20}
= P{a/20 - 81/2 < Y - 81/2 < b/20 - 81/2}
= P{(a/20 - 81/2)/d < (Y - 81/2)/d < (b/20 - 81/2)/d}
~= G[(b/20 - 81/2)/d] - G[(a/20 - 81/2)/d]
比如,a = 210, b = 695时
(b/20 - 81/2)/d
= (695/20 - 81/2)/d
= (695 - 810)/(20d)
= -115/(20d)
= -1.1135697130670763130807031314964
(a/20 - 81/2)/d
= (210/20 - 81/2)/d
= (210 - 810)/(20d)
= -600/(20d)
= -5.8099289377412677204210598165028
P{210 < 从1~80任意取20个数字的和 < 695}
~= G(-1.1135697130670763130807031314964) - G(-5.8099289377412677204210598165028)
= G(5.8099289377412677204210598165028) - G(1.1135697130670763130807031314964)
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