1/(2n-1)(2n+1)=[1/(2n-1)-1/(2n+1)]/2.所以lim[1/3.5+1/5.7.+1/(2n-1)(2n+1)]=lim[1/3-1/5+1/5-1/7+1/7-1/9……+1/(2n-1)-1/(2n+1)]/2=lim[1/3-1/(2n+1)]/2=lim[1/6-1/(4n+2)]=1/6