求和Sn=1^2
(2n-1)^2-(2n)^2=[(2n-1)+2n]*[(2n-1)-2n] =(4n-1)*(-1) =1-4n 所以：Sn=(1-4*1)+(1-4*2)+……+(1-4n) =(1+1+……+1)-4*(1+2+……+n) =n-4*[n*(n+1)/2] =n-2n*(n+1) =-2n^2-n.