利用 (n+1) C(n,k) / (k+1) = C(n+1,k+1)
原式 = 1/(n+1) [ C(n+1,1) - C(n+1,2) +C(n+1,3)-.+(-1)^n C(n+1,n+1) ]
= 1/(n+1) [ 1 - (1-1)^(n+1) ]
= 1/(n+1)