回复：回复：explain pls:)

It is illustrative to show the kth draw's probability is a/(a+b).
Method 1: sample space S=(w1,w2,...wa;r1,r2,...rb),E=(w1,w2...wa) P(E)=C(E)/C(S)=a/a+b
Method 2: taking k balls means a permatation from P(a+b,k).The last is white, number is (a/(a+b))*P(a+b,k).So,P(E)=(a/(a+b))*P(a+b,k)/P(a+b,k)=a/a+b.