−n2(n+1)-\frac{n}{2}(n+1)−2n(n+1)
n2(n+1)\frac{n}{2}(n+1)2n(n+1)
−n(n+1)-n(n+1)−n(n+1)
n(n+1)4\frac{n(n+1)}{4}4n(n+1)