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Solve the given problems. $$\text { Evaluate } \lim _{x \rightarrow 0} \frac{\sin x-x}{x^{3}} \text { by using the expansion for } \sin x$$

Short Answer

Expert verified
The limit is \(-\frac{1}{6}\).

Step by step solution

01

Understanding the Problem

We need to evaluate the limit \( \lim_{x \rightarrow 0} \frac{\sin x - x}{x^3} \). To do this, we'll use the Taylor series expansion for \( \sin x \). Recall that the Taylor series for \( \sin x \) around \( x = 0 \) is \( \sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots \).
02

Substitute the Series into the Expression

Substitute the Taylor series expansion of \( \sin x \) into the expression: \( \frac{\sin x - x}{x^3} = \frac{x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots - x}{x^3} \). Simplifying inside the numerator gives \( \frac{-\frac{x^3}{3!} + \frac{x^5}{5!} - \cdots}{x^3} \).
03

Cancel and Simplify the Expression

Simplify the expression by canceling the common factor of \( x^3 \) in the numerator and the denominator: \( \frac{-\frac{x^3}{6} + \frac{x^5}{120} - \cdots}{x^3} = -\frac{1}{6} + \frac{x^2}{120} - \cdots \). As \( x \rightarrow 0 \), the higher powers of \( x \) approach zero.
04

Evaluate the Limit

Take the limit as \( x \rightarrow 0 \): \( \lim_{x \rightarrow 0} \left(-\frac{1}{6} + \frac{x^2}{120} - \cdots\right) = -\frac{1}{6} \). The terms involving \( x^2 \) and higher powers vanish as \( x \rightarrow 0 \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Taylor Series
In calculus, the Taylor series is a powerful tool used to approximate functions. It provides a series expansion of a function around a specific point. For instance, the Taylor series expansion of a function \( f(x) \) around \( x = a \) is given by:\[f(x) = f(a) + f'(a)(x-a) + \frac{f''(a)}{2!}(x-a)^2 + \frac{f'''(a)}{3!}(x-a)^3 + \cdots\]The series continues indefinitely, capturing more characteristics of the original function with each additional term.
  • Important Points:
    • Convergence: A Taylor series might not converge everywhere but usually does near the point \( a \).
    • General Use: Most useful when functions are hard to compute directly, but you want to evaluate them around a small interval.
    • Calculus Application: Provides a simple way to evaluate limits, derive differential equations, and more.
Sine Function Expansion
The sine function \( \sin x \) can be expanded as a Taylor series around 0, known as the Maclaurin series. The Maclaurin series takes the form:\[\sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots\]This series is very handy when calculating limits or integrals involving \( \sin x \).
  • Utility:
    • Evaluating Limits: When working with indeterminate forms like \( \lim_{x \to 0} \frac{\sin x}{x} \).
    • Approximation: Offers a way to compute \( \sin x \) quickly when \( x \) is small.
  • Characteristics: The series alternates and decreases in magnitude, which helps with convergence.
Evaluating Limits
Evaluating limits is a fundamental concept in calculus that often involves simplifying an expression as a variable approaches a certain value. When direct substitution in a limit results in an indeterminate form like \( \frac{0}{0} \), special techniques such as L'Hôpital's Rule or series expansion can be used.
For example, in the problem \( \lim_{x \rightarrow 0} \frac{\sin x - x}{x^3} \):
  • Use the expansion \( \sin x = x - \frac{x^3}{6} + \frac{x^5}{120} - \cdots \)
  • Simplify: Substitute and simplify the expression to \( -\frac{1}{6} + \frac{x^2}{120} \).
  • Limit Evaluation: As \( x \to 0 \), higher power terms vanish, resulting in the limit value \( -\frac{1}{6} \).

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