这是一个数列求和的问题可以带入等差数列求和公式Sn=n(a1+an)/2 =(2N-3)*(1+2N-3)/2
=2n^2-2n
1+2+3+……+2n-3=(1+(2n-3))(2n-3)/2=2n^2-5n+3
=[1+(2n-3)](2n-3)/2=2n^2-5n+3
(1+2n-3)/2*(2n-3)=(n-1)*(2n-3)
