Give a combinatorial argument (that is, not using algebra, but with an argument about \"choosing\" things) for the identity: C(n+1,k)= (n,k) + (n,k-1). Solution C(n+1,k) = choosing k objects out of (n+1) objects You can choose the first object and k-1 object out of remaining n objects: C(n,k-1) Or you can remove the first object and choose all k objects from the remaining n objects: C(n,k) Therefore: C(n+1,k) = C(n,k) + C(n,k-1).