got the answer from 统计专家Commentate。 看来我是正确的。

explain from Commentate:
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)

so the answer is : a/(a+b)

所有跟帖： 

• 这个推导说明：每次取白球的可能性是一样的。thus,等可能draw。 -戏雨飞鹰- ♀ (0 bytes) () 01/11/2010 postreply 09:00:57

• sweetie you are smart! -bearsback- ♂ (0 bytes) () 01/11/2010 postreply 09:08:44

• 谢谢戏雨飞鹰 -陶陶乐,乐陶陶- ♀ (210 bytes) () 01/11/2010 postreply 11:04:05