9. 某一遞迴函數為 f (n,m) = f (n -1,m) + f (n,m -1)，若 f (n,0) =1, ∀n ≥ 0 且 f (0,m) = 0, ∀m ≥ 0，則 f (2,1) = ___( 10 )___