你可以这样看
先算1/(2n-1)-1/(2n+1)=2n+1/[(2n-1)(2n+1)]-2n-1/[(2n-1)(2n+1)]=
=(2n+1)-(2n-1)]/[(2n-1)(2n+1)]=2/[(2n-1)(2n+1)]
所以1/[(2n-1)(2n+1)]=1/2 *[1/(2n-1)-1/(2n+1)]
同理
因为1/(3n-1)-1/(3n+1)=(3N+1)-(3N-1)/[(3n-1)(3n+1)]=2/[(3n-1)(3n+1)]
所以1/[(3n-1)(3n+1)]=1/2~