As a last part of a solution of a second year algebra problem, I have to prove the identity:

a!b! | (a+b)!

a factorial times b factorial divides a+b factorial.

Anyone have any thoughts on how to tackle it? I tried 'writing out' the (a+b)! part, then cutting of either tail of a! or b!, but to no avail....

Thanks in advance~!

Tijmen