回复：回复：请教解数学题。

来源: 于 2010-09-07 19:44:24 [旧帖] [给我悄悄话] 本文已被阅读：次

f($n,1) should be 1, which is a typo above.

We can see the formula after a few steps. Here is the trick.
f($n,1)=(n+1-1) choose 0 = 1
f($n,2)=(n+2-1) choose 1= n+1
f($n,3)=(n+3-1) choose 2 = (n+2)(n+1)/2
f($n,4)=(n+4-1) choose 3 = (n+3)(n+2)(n+1)/6

The answer to your second question should be (20)(19)(18)(17)(16)/24.

Am I wrong?