1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Does this series converge or diverge?

  1. Aug 17, 2012 #1
    1. The problem statement, all variables and given/known data
    does the following series converge or diverge? how does one determine whether it diverges or converges?

    2. Relevant equations
    Ʃ (n+1)/((n^3)+3n^2+5)^1/2

    n=1, to infinity

    3. The attempt at a solution
    I attempted to compare it with n/(n)^3/2, this series diverges and is greater than the original series, so the comparison test didn't work, at least in this case. the ratio test was inconclusive as well. any help would be appreciated, thanks
    Last edited: Aug 17, 2012
  2. jcsd
  3. Aug 17, 2012 #2
    Try using the asymptotic comparison test. I'll write it down in case you don't know it.

    If [itex]|a_{n}|[/itex] ~ [itex]|b_{n}|[/itex]


    [itex]\sum |a_{n}|[/itex] converges [itex]\Leftrightarrow[/itex] [itex]\sum |b_{n}|[/itex] converges

    ~ means "is asymptotic to".

    [itex]a_{n}[/itex] ~ [itex]b_{n} \Leftrightarrow [/itex] limit [itex]{a_{n}}/{b_{n}}\rightarrow 1[/itex]

    It's a very very useful test.
  4. Aug 17, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Dansuer's suggestion is good, but you probably know it as the limit comparison test. The limit has to be non-zero and finite, not necessarily equal to 1.
  5. Aug 17, 2012 #4


    User Avatar
    Science Advisor

    The crucial point is that the denominator has "leading power" 3/2 while the numerator has power 1. The entire fraction has "power" -1/2. You should be able to compare it with [itex]1/n^{1/2}[/itex].
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook