极限lim n(2n-1)分之1+2+3+...+n=

问题描述:

极限lim n(2n-1)分之1+2+3+...+n=

=lim n(n+1)/2n(2n-1)
=lim (n+1)/(4n-2)
=1/4lim n(n+1)/2n(2n-1)这一步怎么弄的啊1+2+3+……+n=n(n+1)/2