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(1)
P(3人都没进) = (1-2/5)(1-1/2)(1-3/5) = (3/5)(1/2)(2/5) = 3/25
(2)
P(3人中恰有2人没进)
=P(只有1人进球)
=(2/5)(1-1/2)(1-3/5) + (1/2)(1-2/5)(1-3/5) + (3/5)(1-2/5)(1-1/2)
=(2/5)(1/2)(2/5) +(1/2)(3/5)(2/5) + (3/5)(3/5)(1/2)
= 19/50
P(3人都没进) = (1-2/5)(1-1/2)(1-3/5) = (3/5)(1/2)(2/5) = 3/25
(2)
P(3人中恰有2人没进)
=P(只有1人进球)
=(2/5)(1-1/2)(1-3/5) + (1/2)(1-2/5)(1-3/5) + (3/5)(1-2/5)(1-1/2)
=(2/5)(1/2)(2/5) +(1/2)(3/5)(2/5) + (3/5)(3/5)(1/2)
= 19/50
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