Question

Sn=1x2/2 + 2x3/2 +3x4/2+.+nx(n+1)/2 = __________

Answer 1

你把1/2提出来,1x2+2x3+...+n(n+1)=n(n+1)(2n+1)/6+n(n+1）/2,n(n+1)=n^2+n,然后拆开,n^2相加n(n+1)(2n+1)/6,n相加n(n+1）/2,就可以了.再把n(n+1)(2n+1)/6+n(n+1）/2除以2就可以了.