已知数列1,2,3,4……2n,求他们的和
问题描述：
已知数列1,2,3,4……2n,求他们的和
答
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