Math Problem Statement

Consider the limit \lim_{x\to\infty}\frac{x+\sin x}{x}. Let f(x)=x+\sin x and g(x)=x. Which of the following statements are Turue?

Question 4Answer

a. g'(x)\neq 0 on (M,\infty) for some M>0

b. f,g are differentiable on (M,\infty) for some M>0

c. \lim_{x\to\infty}\frac{f(x)}{g(x)} does not exist

d. f,g are continuous on (M,\infty) for some M>0

e. \lim_{x\to\infty}\frac{f'(x)}{g'(x)} does not exist

f. \lim_{x\to\infty}\frac{f'(x)}{g'(x)} exists

g. \lim_{x\to\infty}\frac{f(x)}{g(x)} is a \frac{\infty}{\infty} form

Solution

Let's evaluate the limit and the given statements carefully.

We are given:

limxx+sinxx\lim_{x \to \infty} \frac{x + \sin x}{x}

and two functions: f(x)=x+sinxf(x) = x + \sin x and g(x)=xg(x) = x.

Statement a. g(x)0g'(x) \neq 0 on (M,)(M, \infty) for some M>0M > 0

  • g(x)=xg(x) = x, so g(x)=1g'(x) = 1.
  • Since g(x)=1g'(x) = 1 is non-zero for all x(M,)x \in (M, \infty), this statement is True.

Statement b. ff and gg are differentiable on (M,)(M, \infty) for some M>0M > 0

  • f(x)=x+sinxf(x) = x + \sin x is differentiable because both xx and sinx\sin x are differentiable.
  • g(x)=xg(x) = x is also differentiable since it's a simple linear function.
  • Thus, both functions are differentiable on (M,)(M, \infty), so this statement is True.

Statement c. limxf(x)g(x)\lim_{x \to \infty} \frac{f(x)}{g(x)} does not exist

Let's compute the limit:

limxx+sinxx=limx(1+sinxx)\lim_{x \to \infty} \frac{x + \sin x}{x} = \lim_{x \to \infty} \left( 1 + \frac{\sin x}{x} \right)

Since sinxx0\frac{\sin x}{x} \to 0 as xx \to \infty, we get:

limxf(x)g(x)=1\lim_{x \to \infty} \frac{f(x)}{g(x)} = 1

This limit exists, so this statement is False.

Statement d. ff and gg are continuous on (M,)(M, \infty) for some M>0M > 0

  • f(x)=x+sinxf(x) = x + \sin x is continuous because both xx and sinx\sin x are continuous functions.
  • g(x)=xg(x) = x is continuous as a linear function.
  • Thus, both ff and gg are continuous on (M,)(M, \infty), making this statement True.

Statement e. limxf(x)g(x)\lim_{x \to \infty} \frac{f'(x)}{g'(x)} does not exist

Let's differentiate f(x)f(x) and g(x)g(x):

  • f(x)=1+cosxf'(x) = 1 + \cos x
  • g(x)=1g'(x) = 1

Now, we compute:

f(x)g(x)=1+cosx\frac{f'(x)}{g'(x)} = 1 + \cos x

Since cosx\cos x oscillates between 1-1 and 11, the expression 1+cosx1 + \cos x oscillates between 00 and 22. Therefore, the limit does not exist because of this oscillation. Thus, this statement is True.

Statement f. limxf(x)g(x)\lim_{x \to \infty} \frac{f'(x)}{g'(x)} exists

From the reasoning above, the limit does not exist due to the oscillating behavior of cosx\cos x, so this statement is False.

Statement g. limxf(x)g(x)\lim_{x \to \infty} \frac{f(x)}{g(x)} is a \frac{\infty}{\infty} form

  • When we initially compute the limit, the form f(x)g(x)=x+sinxx\frac{f(x)}{g(x)} = \frac{x + \sin x}{x} is indeed an indeterminate form \frac{\infty}{\infty} because both the numerator and denominator go to infinity as xx \to \infty.
  • Therefore, this statement is True.

Summary of True/False:

  • a. True
  • b. True
  • c. False
  • d. True
  • e. True
  • f. False
  • g. True

Would you like more details on any of these steps or explanations?

Here are 5 related questions:

  1. How do you apply L'Hôpital's Rule to limits involving \frac{\infty}{\infty} forms?
  2. What is the behavior of trigonometric functions like sinx\sin x and cosx\cos x as xx \to \infty?
  3. How does the differentiability of functions affect their limits?
  4. What are the conditions for the existence of limits involving oscillating functions?
  5. What does the term "indeterminate form" mean in calculus?

Tip: When limits are in indeterminate forms like \frac{\infty}{\infty}, consider applying L'Hôpital's Rule to simplify the expression.

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Math Problem Analysis

Mathematical Concepts

Calculus
Limits
Indeterminate Forms
Differentiability
Continuity

Formulas

\lim_{x \to \infty} \frac{x + \sin x}{x} = 1 + \frac{\sin x}{x}
\frac{f'(x)}{g'(x)} = 1 + \cos x

Theorems

L'Hôpital's Rule
Limit Theorem

Suitable Grade Level

Grades 11-12 and early university