Proving Unboundedness of {xn} with [(n+1)/n]^3 - n^3

  • Thread starter Thread starter sara_87
  • Start date Start date
  • Tags Tags
    Sequence
Click For Summary
SUMMARY

The sequence {xn} defined by the expression [(n+1)/n]^3 - n^3 is proven to be unbounded. By analyzing the function f(x) = [(x+1)/x]^3 - x^3, it is established that f is monotonically decreasing for all x > 0. Consequently, the range of f is (0, infinity) for x > 0, confirming that for any M > 0, there exists an N such that |xN| > M, thereby demonstrating the unboundedness of the sequence.

PREREQUISITES
  • Understanding of sequences and their properties
  • Knowledge of calculus, specifically derivatives and monotonic functions
  • Familiarity with limits and infinity in mathematical analysis
  • Basic algebraic manipulation skills
NEXT STEPS
  • Study the properties of monotonic functions in calculus
  • Learn about sequences and series in mathematical analysis
  • Explore the concept of limits and unbounded sequences
  • Investigate the application of derivatives in proving function behavior
USEFUL FOR

Mathematics students, particularly those studying calculus and real analysis, as well as educators seeking to understand the properties of sequences and functions.

sara_87
Messages
748
Reaction score
0

Homework Statement



show that the following sequence is unbounded

{xn}= [(n+1)/n]^3 - n^3

Homework Equations





The Attempt at a Solution



I expanded it but didnt get anywhere after that. I know that a sequence is unbounded if for all M>0 there exists N such that abs(xN)>M but i still don't understand how to show it.
 
Physics news on Phys.org
Here's something to try: Consider the following function of a real variable.

f(x)=[(x+1)/x]^3-x^3

Now take the derivative and show that f is monotonically decreasing for all x>0, and that the range of f is (0,infinity) when x>0. (sorry, LaTeX is acting funny)

What then can you conclude about f(n)?
 

Similar threads

  • · Replies 17 ·
Replies
17
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
Replies
20
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K