Explaining Convergence of (3^n)/(2^n + 4^n) w/o Limit Comp. Test

In summary, the conversation discusses using the comparison test to show that the series (3^n)/(2^n + 4^n) is convergent, without using the limit comparison test. The attempt at a solution involves comparing the series to 1/2^n, but the issue of removing the 3^n on the top arises. Suggestions are made to delete terms in the denominator and rewrite the inequality to obtain 3n/(2n + 4n) for the left side.
  • #1
Erind
2
0

Homework Statement



Use the comparison test to explain why the series (3^n)/(2^n + 4^n) is convergent. He said specifically not to use the limit comparison test on this one.

Homework Equations



1/2^n

The Attempt at a Solution



I know I should be comparing it to 1/2^n because it is a geometric series and thereby convergent, but I can't get rid of the 3^n on the top in order to get there.
 
Physics news on Phys.org
  • #2
Ok, I just came up with comparing (3^n)/(2^n + 4^n) < (3^n)/(2^n + 4^n)(3^n) and then canceling the (3^n) and then 1/(2^n + 4^n) < 1/(2^n) and then it's done, but I'm not sure if that first inequality is a legal move.
 
  • #3
No, I'm afraid that it isn't a legal move :frown:

Maybe you should delete some of the terms in the denumerator...
 
  • #4
Try starting with 2n + 4n > 4n, then rewrite the inequality until you get 3n/(2n + 4n) for the left side of the inequality.
 

1. What is the formula for (3^n)/(2^n + 4^n)?

The formula for (3^n)/(2^n + 4^n) is an exponential function. It can also be written as (3/2)^n / (1 + (2/4)^n).

2. How do you explain the convergence of (3^n)/(2^n + 4^n)?

The convergence of (3^n)/(2^n + 4^n) can be explained by using the ratio test. As n approaches infinity, the ratio between consecutive terms approaches a finite value, which indicates convergence.

3. Why is the limit comparison test used to explain the convergence of (3^n)/(2^n + 4^n)?

The limit comparison test is used because it allows us to compare the given series to a known series with known convergence properties. This helps us determine the convergence of the given series without having to evaluate its individual terms.

4. What is the limit comparison test for (3^n)/(2^n + 4^n)?

The limit comparison test for (3^n)/(2^n + 4^n) involves taking the limit of the ratio of the given series to a known convergent series. If the limit is a nonzero finite value, then the given series also converges.

5. Can you explain the steps for using the limit comparison test to prove the convergence of (3^n)/(2^n + 4^n)?

To use the limit comparison test for (3^n)/(2^n + 4^n), we first need to choose a known convergent series to compare it to. Then, we take the limit as n approaches infinity of the ratio of the two series. If the limit is a nonzero finite value, then we can conclude that the given series converges. If the limit is zero or infinity, then the convergence of the given series cannot be determined using this test.

Similar threads

  • Calculus and Beyond Homework Help
Replies
14
Views
1K
  • Calculus and Beyond Homework Help
Replies
5
Views
475
  • Calculus and Beyond Homework Help
Replies
14
Views
1K
Replies
29
Views
4K
  • Calculus and Beyond Homework Help
Replies
5
Views
1K
  • Calculus and Beyond Homework Help
Replies
17
Views
1K
  • Calculus and Beyond Homework Help
Replies
16
Views
3K
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
  • Calculus and Beyond Homework Help
Replies
7
Views
943
  • Calculus and Beyond Homework Help
Replies
11
Views
1K
Back
Top