S summergrl New member Joined Feb 21, 2007 Messages 33 Apr 10, 2007 #1 Test the series for convergence or divergence. I think we are supposed to be using the alternating series test. -1/3 + 2/4 - 3/5 + 4/6 - 5/7 + ...
Test the series for convergence or divergence. I think we are supposed to be using the alternating series test. -1/3 + 2/4 - 3/5 + 4/6 - 5/7 + ...
G galactus Super Moderator Staff member Joined Sep 28, 2005 Messages 7,203 Apr 10, 2007 #2 Check the two conditions of the alternating series. Your series: \(\displaystyle \L\\\displaystyle\sum_{n=1}^{\infty}(-1)^{n}\frac{n}{n+2}\) Is it true that: \(\displaystyle \L\\a_{1} \;\ > \;\ a_{2} \;\ > \;\ a_{3} \;\ > \;\ .... \;\ > \;\ a_{k} \;\ > \;\ ....\) Check the limit: \(\displaystyle \L\\\lim_{k\to\infty}\frac{n}{n+2}\). Does it equal 0?.
Check the two conditions of the alternating series. Your series: \(\displaystyle \L\\\displaystyle\sum_{n=1}^{\infty}(-1)^{n}\frac{n}{n+2}\) Is it true that: \(\displaystyle \L\\a_{1} \;\ > \;\ a_{2} \;\ > \;\ a_{3} \;\ > \;\ .... \;\ > \;\ a_{k} \;\ > \;\ ....\) Check the limit: \(\displaystyle \L\\\lim_{k\to\infty}\frac{n}{n+2}\). Does it equal 0?.
S summergrl New member Joined Feb 21, 2007 Messages 33 Apr 10, 2007 #3 No, its increasing and the limit is one.
G galactus Super Moderator Staff member Joined Sep 28, 2005 Messages 7,203 Apr 10, 2007 #4 Good. Now, does that mean it diverges or converges?.
G galactus Super Moderator Staff member Joined Sep 28, 2005 Messages 7,203 Apr 10, 2007 #6 Very good. You get a cookie.
S summergrl New member Joined Feb 21, 2007 Messages 33 Apr 10, 2007 #7 thanks! (-1) ^n sqrt(n)/1+2sqrt(2) does this one diverge too?
G galactus Super Moderator Staff member Joined Sep 28, 2005 Messages 7,203 Apr 10, 2007 #8 I believe so. Check it the same way.