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Infinite series convergence II

cacophony

New member
Apr 17, 2013
4
Test these for convergence.

3.
infinity
E........((-1)^n)*(n^3 + 3n)/((n^2) + 7n)
n = 2

4.
infinity
E........ln(n^3)/n^2
n = 2

note: for #3: -((n^2 + 3n))/n) is all to the power of e

Btw, E means sum.

Which tests should I use to solve these?
 
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chisigma

Well-known member
Feb 13, 2012
1,704
3. $\displaystyle \sum_{n=2}^{\infty} (-1)^{n} \frac{n^{3} + 3\ n}{n^{2} + 7\ n}$
What is $\displaystyle \lim_{n \rightarrow \infty} (-1)^{n} \frac{n^{3} + 3\ n}{n^{2} + 7\ n}$?...

Kind regards

$\chi$ $\sigma$
 

ZaidAlyafey

Well-known member
MHB Math Helper
Jan 17, 2013
1,667

Prove It

Well-known member
MHB Math Helper
Jan 26, 2012
1,403
A necessary (though not sufficient) condition for a series to converge is that the terms in the series eventually have to vanish to 0. That means that if they do NOT vanish to 0, the series is divergent.

So what happens to the terms in the first series as you go along?
 

Barioth

Member
Jan 17, 2013
52
For the number 4
\(\displaystyle \sum_2^\infty\frac{\ln{n^3}}{n^2}\)

I would try the good old Comparison test!

Give it a try !
 

ZaidAlyafey

Well-known member
MHB Math Helper
Jan 17, 2013
1,667
For number 4

\(\displaystyle \ln(n) < \sqrt{n} \)