How can the limit of a function be determined without using a graph?

  • Thread starter Thread starter BayernBlues
  • Start date Start date
  • Tags Tags
    Limits
Click For Summary
SUMMARY

The limit of the function does not exist at x=2, as established through mathematical evaluation rather than graphical representation. By substituting values close to 2, it is evident that the function approaches positive infinity from the right (x -> 2+) and does not converge from the left (x -> 2-). The analysis involves evaluating the behavior of the numerator, specifically the expression x^3 + 3x^2 + 4, which exceeds 8 as x approaches 2 from the right. This confirms that the limit diverges to positive infinity.

PREREQUISITES
  • Understanding of limits in calculus
  • Familiarity with asymptotic behavior of functions
  • Knowledge of polynomial functions and their properties
  • Ability to perform sign analysis on intervals
NEXT STEPS
  • Study the concept of limits and continuity in calculus
  • Learn about asymptotes and their implications on function behavior
  • Explore polynomial function analysis techniques
  • Practice sign graphs and their use in determining limits
USEFUL FOR

Students studying calculus, particularly those focusing on limits and function behavior, as well as educators seeking to enhance their teaching methods in mathematical analysis.

BayernBlues
Messages
61
Reaction score
0

Homework Statement



http://img442.imageshack.us/img442/7875/pset33po8.png

Homework Equations





The Attempt at a Solution



I know that the answer is limit does not exist but I don't know how to show it mathematically on paper. Is there a way to show this without making a graph?
 
Last edited by a moderator:
Physics news on Phys.org
Plug in 2. You get a nonzero number over 0, which means dividing a common factor won't help. The graph is asymtotic at x=2. You can show which infinity the function goes to with a sign graph. Find all important numbers (roots, asymtotes) and evaluate the function in between each number. The graph will only change signs possibly at a root or asymtote. By plugging in values close to 2 on either side. You should see that the limit is split and does not exist
 
A quick way to check this is to see how does the numerator behave with x -> 2+ and x -> 2-, i.e.

2^3 + 3*2^2 + 4 = 8. Obviously, if x - > 2+, x^3 + 3x^2 + 4 > 8. Now,

x^3 + 3x^2 + 4 > 8

x^3 + 3x^2 + 4 / x - 2 > 8 / x - 2

Since on the right hand side the limit is + infinity, by the inequality we conclude that the limit on left is also + infinity. Now you can proceed with 2-... it may seem a bit long on paper, but it easily done mentally.
 

Similar threads

  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
Replies
4
Views
2K
  • · Replies 6 ·
Replies
6
Views
4K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 4 ·
Replies
4
Views
3K
Replies
3
Views
1K
Replies
10
Views
2K