n^2=n(n-1)+n=2c(n,2)+n,
原式=2[c(2,2)+c(3,2)+……+c(n,2)]+(1+2+……+n)
=2[c(3,3)+c(3,2)+……+c(n,2)]+n(1+n)/2
=2c(n+1,3)+n(1+n)/2
=(n+1)n(n-1)/3+n(1+n)/2
=[n(n+1)(2n+1)]/6.
n^2=n(n-1)+n=2c(n,2)+n,
原式=2[c(2,2)+c(3,2)+……+c(n,2)]+(1+2+……+n)
=2[c(3,3)+c(3,2)+……+c(n,2)]+n(1+n)/2
=2c(n+1,3)+n(1+n)/2
=(n+1)n(n-1)/3+n(1+n)/2
=[n(n+1)(2n+1)]/6.