回复：explain pls:)
Commentate
2010-01-11 08:38:10
( reads)
It is illustrative to show the 2nd draw's probability is a/(a+b).
Obviously, the first draw P(white) = a/(a+b), P(red) = b/(a+b).
now the 2nd draw:
P(white) = P(1st draw white)*P(2nd draw white) + P(1st draw red)*P(2nd draw white)
= a/(a+b) * (a-1)/(a+b-1) + b/(a+b) * a/(a+b-1) = a/(a+b)
戏雨飞鹰
2010-01-11 08:42:26wow! 谢谢专家。