Math Problem Statement

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Prove that the limit of (3n^2 + 2n - 5) / (2n^2 - 5) as n approaches infinity is 3/2.

Solution

To prove the given limit, we will focus on the expression:

limn+3n2+2n52n25=32\lim_{n \to +\infty} \frac{3n^2 + 2n - 5}{2n^2 - 5} = \frac{3}{2}

Step-by-Step Proof:

  1. Focus on the highest powers of nn: When nn approaches infinity, terms with the highest power of nn dominate. The highest power of nn in both the numerator and denominator is n2n^2.

  2. Factor out n2n^2 from both the numerator and the denominator:

    3n2+2n52n25=n2(3+2n5n2)n2(25n2)\frac{3n^2 + 2n - 5}{2n^2 - 5} = \frac{n^2(3 + \frac{2}{n} - \frac{5}{n^2})}{n^2(2 - \frac{5}{n^2})}

  3. Simplify the expression by canceling out the n2n^2 terms:

    =3+2n5n225n2= \frac{3 + \frac{2}{n} - \frac{5}{n^2}}{2 - \frac{5}{n^2}}

  4. Take the limit as nn \to \infty: As nn becomes infinitely large, both 2n\frac{2}{n} and 5n2\frac{5}{n^2} tend to 0. Thus, we are left with:

    =3+0020=32= \frac{3 + 0 - 0}{2 - 0} = \frac{3}{2}

Hence, the limit is indeed:

32\boxed{\frac{3}{2}}

Conclusion:

The limit is correctly evaluated as 32\frac{3}{2}, and the steps confirm the given statement.


Would you like more details on any specific step?

Here are five related questions to consider:

  1. What happens if the degree of the numerator is lower than that of the denominator?
  2. How would you handle limits if the degree of the numerator were higher than the denominator?
  3. How do we handle limits when there's an indeterminate form like \frac{\infty}{\infty}?
  4. What is the general rule for limits of rational functions at infinity?
  5. How can L'Hôpital's rule be applied to similar problems?

Tip: When calculating limits at infinity for rational functions, always compare the degrees of the numerator and denominator to determine the behavior.

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Math Problem Analysis

Mathematical Concepts

Limits
Rational Functions
Asymptotic Behavior

Formulas

lim_{n -> ∞} (an^m + ... + c) / (bn^m + ... + d) = a / b when m is the highest degree

Theorems

Limit of rational functions as n approaches infinity
Degree of numerator vs. denominator in limits

Suitable Grade Level

Grades 11-12