How to Solve a Limit Problem Involving Logarithms?

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Discussion Overview

The discussion revolves around the evaluation of a limit involving logarithms, specifically the limit ##\lim_{n \to \infty} \frac{n^{1.74}}{n \times (\log n)^9}##. Participants explore the validity of taking logarithms of both the numerator and denominator, and the implications of such transformations on the limit's evaluation.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that taking logs of both the numerator and denominator leads to the limit ##=\lim_{n \to \infty} \frac{1.74 \times \log n}{\log n + 9 \log \log n}##.
  • Another participant challenges this approach, arguing that taking the logarithm of a quotient does not yield a quotient of logs, and emphasizes that the transformation changes the nature of the limit.
  • Some participants suggest that logarithmic transformations can be useful for comparing growth rates of functions, though this is contested.
  • A request for clarification on the educational level of the problem is made, indicating uncertainty about whether it is suitable for high school or university students.

Areas of Agreement / Disagreement

Participants express disagreement regarding the validity of taking logarithms of both the numerator and denominator in the limit evaluation. There is no consensus on the correct approach to solving the limit.

Contextual Notes

Participants highlight the importance of understanding the properties of logarithms, particularly in relation to products and quotients, and the assumptions required for their application. The discussion reflects uncertainty about the mathematical steps involved in the limit evaluation.

22990atinesh
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Suppose there is a limit
##\lim_{n \to \infty} \frac{n^{1.74}}{n \times (\log n)^9}##
Taking logs both on numerator and denominator
##=\lim_{n \to \infty} \frac{1.74 \times \log n}{\log n + 9 \log \log n}##
What can we say about the limit as n approaches ##\infty##
 
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How can you take log on both numerator and denominator. You are changing the question.
1/2= 0.5 is not the same as
log(1)/log(2) , which is 0.
 
Raghav Gupta said:
How can you take log on both numerator and denominator. You are changing the question.
1/2= 0.5 is not the same as
log(1)/log(2) , which is 0.
In order to compare growth rate of two functions we can do that.
 
Can you give an example?
Algebraically , in question you have stated how one can do that on numbers?
Is that a university( college ) level problem
Or a high school one?
 
22990atinesh said:
Suppose there is a limit
##\lim_{n \to \infty} \frac{n^{1.74}}{n \times (\log n)^9}##
Taking logs both on numerator and denominator
##=\lim_{n \to \infty} \frac{1.74 \times \log n}{\log n + 9 \log \log n}##
What can we say about the limit as n approaches ##\infty##

22990atinesh said:
In order to compare growth rate of two functions we can do that.
In your first post (quoted above) you are claiming that ##\lim_{n \to \infty} \frac{n^{1.74}}{n \times (\log n)^9} = \lim_{n \to \infty} \frac{1.74 \times \log n}{\log n + 9 \log \log n}##. Most emphatically, this is NOT TRUE! You can take the log of both sides of an equation, but you cannot take the log of a quotient to get a quotient of logs, nor can you take the log of a sum and get the sum of the logs.

What IS true is that log(a * b) = log(a) + log(b), and that log(a/b) = log(a) - log(b), assuming suitable restrictions on the values of a and b.
 

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