(12)n(n−1)\left(\frac{1}{2}\right) n(n-1)(21)n(n−1)
n(n−1)n(n-1)n(n−1)
(12)(n−1)\left(\frac{1}{2}\right)(n-1)(21)(n−1)
(12)n(n+1)\left(\frac{1}{2}\right) n(n+1)(21)n(n+1)
