C clw89 New member Joined Jan 23, 2008 Messages 6 Mar 20, 2008 #1 determine whether the series is convergent or divergent: the series from n=1 to infinity of ln(n)/n[sup:36no4ihc]3[/sup:36no4ihc] Thanks!!!
determine whether the series is convergent or divergent: the series from n=1 to infinity of ln(n)/n[sup:36no4ihc]3[/sup:36no4ihc] Thanks!!!
G galactus Super Moderator Staff member Joined Sep 28, 2005 Messages 7,203 Mar 20, 2008 #2 Try using the integral test. \(\displaystyle \int_{1}^{L}\frac{ln(n)}{n^{3}}=\frac{-ln(L)}{2L^{2}}-\frac{1}{4L^{2}}+\frac{1}{4}\) Now, take the limit: \(\displaystyle \lim_{L\to\infty}\left[\frac{-ln(L)}{2L^{2}}-\frac{1}{4L^{2}}+\frac{1}{4}\right]\) See the limit?. What does that tell you about convergence?.
Try using the integral test. \(\displaystyle \int_{1}^{L}\frac{ln(n)}{n^{3}}=\frac{-ln(L)}{2L^{2}}-\frac{1}{4L^{2}}+\frac{1}{4}\) Now, take the limit: \(\displaystyle \lim_{L\to\infty}\left[\frac{-ln(L)}{2L^{2}}-\frac{1}{4L^{2}}+\frac{1}{4}\right]\) See the limit?. What does that tell you about convergence?.
pka Elite Member Joined Jan 29, 2005 Messages 11,978 Mar 20, 2008 #3 Because \(\displaystyle \ln(n) < n\) direct comparison is faster.