Question

已知数列1,2,3,4……2n,求他们的和

Answer 1

1,+2+3+4+……+2n

=(1+2n)+(2+2n-1)+(3+2n-2)+……+(n+2n-n)

=(1+2n)(n-1)+n

=n-1+2n^2-2n+n

=2n^2 -1