Math Problem Statement

Show by means of an example that \( \lim_{x \to a} \left[ f(x) g(x) \right] \) may exist even though neither \( \lim_{x \to a} f(x) \) nor \( \lim_{x \to a} g(x) \) exists.

Solution

To solve this problem, we need to find an example where the limit of the product of two functions f(x)f(x) and g(x)g(x) as xax \to a exists, even though the individual limits limxaf(x)\lim_{x \to a} f(x) and limxag(x)\lim_{x \to a} g(x) do not exist.

Example:

Consider the functions: f(x)=sin(1x)andg(x)=1xf(x) = \sin\left(\frac{1}{x}\right) \quad \text{and} \quad g(x) = \frac{1}{x} as x0x \to 0.

  1. Examining f(x)=sin(1x)f(x) = \sin\left(\frac{1}{x}\right):

    • As x0x \to 0, the argument 1x\frac{1}{x} oscillates between positive and negative infinity, causing sin(1x)\sin\left(\frac{1}{x}\right) to oscillate between -1 and 1.
    • Therefore, limx0sin(1x)\lim_{x \to 0} \sin\left(\frac{1}{x}\right) does not exist because of the oscillations.
  2. Examining g(x)=1xg(x) = \frac{1}{x}:

    • As x0x \to 0, 1x\frac{1}{x} diverges to either ++\infty or -\infty, depending on the direction from which xx approaches 0.
    • Thus, limx01x\lim_{x \to 0} \frac{1}{x} also does not exist.
  3. Examining the product f(x)g(x)=sin(1x)1xf(x) \cdot g(x) = \sin\left(\frac{1}{x}\right) \cdot \frac{1}{x}:

    • We analyze limx0sin(1x)1x\lim_{x \to 0} \sin\left(\frac{1}{x}\right) \cdot \frac{1}{x}.
    • Using the fact that 1sin(1x)1-1 \leq \sin\left(\frac{1}{x}\right) \leq 1, we can bound the product: 1xsin(1x)1x1x-\frac{1}{x} \leq \sin\left(\frac{1}{x}\right) \cdot \frac{1}{x} \leq \frac{1}{x}
    • However, as x0x \to 0, this expression behaves more gently. Specifically: [ \lim_{x \to

Ask a new question for Free

By Image

Drop file here or Click Here to upload

Math Problem Analysis

Mathematical Concepts

Limits
Product of Limits
Oscillatory Functions
Limit Laws

Formulas

\( \lim_{x \to a} \left[ f(x) \cdot g(x) \right] \)

Theorems

Limit of a product theorem (special cases)

Suitable Grade Level

College Calculus