[求助] what should be the solution for this probability question?

lilly58

The question:a whilte balls and b red balls.  If we take one ball
at a time without putting them back into the bag, what's the
probability that we could get a white ball at the Kth time?

octopus555

这个……是托福的题吗？

好复杂哦……

sony

k = a +1 ,　我瞎做的，呵呵，请牛人来做吧～

不误导祖国的花朵了

hoodzhj

如果前k-1次都没有抽中白球，则第k次的可能性是b/(a+b-k)

[此贴子已经被作者于2004-4-9 5:28:38编辑过]

annie919

mm, 这题你有答案吗？都忘了，有一些数学符号，我电脑上没有，不知对不对？

P=  [ a*（a+b-1）!/(a-1)!]/[（a+b）!-(k-1)!]