分裂开：
∑1/[(2n+1)(2n-1)]
=1/2[1-1/3+1/3-1/5+1/5-1/7+...+1/(2n-1)-1/(2n+1)]
=1/2[1-1/(2n+1)]
=n/(2n+1)所以和是1/3对吧怎么是1/3呢，是1/2当n趋向∞，n/(2n+1)=1/(2+1/n)=1/2