For n≥2,nCr=Cr and an=∑r=0n 1Cr

P ∑r=0nr nCr=∑r=0nn−r nCn−r=∑r=0nn nCr-∑r=0nr nCr    ⇒2∑γ=0n rCr=∑r=0nn nCr=n2an&     r nCr=n n−1Cr−1R     ∑             r=1              n 1r nCr=∑r=1n 1n n−1Cr−1=1n∑r=0n-1 1 n−1Cr=1nan−1S                ∑                            r=0                             n−1 1(n−r)·Cr=∑r=0n−1 1(n−r)·nCn-r=1nan−1