Math Problem Statement

lim x to inf (xln(1-(5/x)+(6/(x^2)) found the limit

Solution

Let's analyze the given limit:

limxxln(15x+6x2)\lim_{x \to \infty} x \ln\left(1 - \frac{5}{x} + \frac{6}{x^2}\right)

Step 1: Simplifying the expression inside the logarithm

As xx \to \infty, the terms 5x\frac{5}{x} and 6x2\frac{6}{x^2} become very small. Therefore, we can approximate:

15x+6x215x1 - \frac{5}{x} + \frac{6}{x^2} \approx 1 - \frac{5}{x}

The term 6x2\frac{6}{x^2} becomes negligible compared to 5x\frac{5}{x} for large xx, so we approximate:

ln(15x+6x2)ln(15x)\ln\left(1 - \frac{5}{x} + \frac{6}{x^2}\right) \approx \ln\left(1 - \frac{5}{x}\right)

Step 2: Using the approximation for ln(1+u)\ln(1 + u) when uu is small

For small values of uu, we can use the approximation:

ln(1+u)uwhenu0\ln(1 + u) \approx u \quad \text{when} \quad u \to 0

Thus, for u=5xu = -\frac{5}{x}, we have:

ln(15x)5x\ln\left(1 - \frac{5}{x}\right) \approx -\frac{5}{x}

Step 3: Substituting this approximation back into the limit

Now, substitute ln(15x)5x\ln\left(1 - \frac{5}{x}\right) \approx -\frac{5}{x} into the original limit:

limxxln(15x+6x2)limxx(5x)\lim_{x \to \infty} x \ln\left(1 - \frac{5}{x} + \frac{6}{x^2}\right) \approx \lim_{x \to \infty} x \left( -\frac{5}{x} \right)

Simplifying:

=limx5=5= \lim_{x \to \infty} -5 = -5

Final Answer:

The limit is 5\boxed{-5}.


Would you like further details or have any other questions?

Here are some related questions you might find interesting:

  1. What is the behavior of logarithmic functions as xx \to \infty?
  2. How can we use series expansions to approximate limits?
  3. What happens to the limit if we have higher-order terms in the approximation?
  4. How do we handle indeterminate forms like 0×0 \times \infty in limits?
  5. Can this result be generalized to other similar expressions involving logarithms?

Tip: Always check whether approximations are valid for large xx before proceeding, especially in limits involving small terms like 1x\frac{1}{x}.

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Math Problem Analysis

Mathematical Concepts

Limits
Logarithms
Approximations for large x
Asymptotic behavior

Formulas

\ln(1 + u) \approx u for small u
lim_{x \to \infty} x \ln(1 - a/x) = -a

Theorems

Limit of logarithmic expressions as x approaches infinity
Approximation of logarithms for small values

Suitable Grade Level

Grades 11-12