1/(2n-1)-1/(2n+1)=[(2n+1)-(2n-1)]/(2n-1)(2n+1)=2/(2n-1)(2n+1)【这是研究方向】于是(9/2)[1/(2n-1)-1/(2n+1)]=9/(2n-1)(2n+1)=(3/2n-1)(3/2n+1)【反过来思考会简单的多】即(3/2n-1)(3/2n+1)=(9/2)[1/(2n-1)-1/(2n+1)]