Using the Sandwich Theorem to Solve Limits Involving Trigonometric Functions

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SUMMARY

The discussion focuses on using the Sandwich Theorem to evaluate the limit of the expression \(\lim_{x \to 0} \sqrt{x^3 + x^2}\sin\frac{\pi}{x}\). Participants confirm that since \(-1 \leq \sin\frac{\pi}{x} \leq 1\), it follows that \(-\sqrt{x^3 + x^2} \leq \sqrt{x^3 + x^2}\sin\frac{\pi}{x} \leq \sqrt{x^3 + x^2}\). As both bounding limits approach 0 as \(x\) approaches 0, the limit of the original expression is also 0. Concerns regarding the undefined nature of \(\frac{\pi}{x}\) at \(x = 0\) are addressed, confirming that the limit process is valid since \(x\) does not equal 0 during the evaluation.

PREREQUISITES
  • Understanding of limits in calculus
  • Familiarity with the Sandwich Theorem
  • Knowledge of trigonometric functions, specifically the sine function
  • Basic algebraic manipulation of expressions involving square roots
NEXT STEPS
  • Study the Sandwich Theorem in detail to understand its applications in limit evaluation
  • Learn about the behavior of trigonometric functions near their undefined points
  • Explore advanced limit techniques, including L'Hôpital's Rule
  • Practice solving limits involving oscillatory functions and their bounds
USEFUL FOR

Students of calculus, mathematics educators, and anyone interested in mastering limit evaluation techniques, particularly in the context of trigonometric functions.

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Homework Statement



\lim_{x \to 0} \sqrt{x^3 + x^2}\sin\frac{\pi}{x}

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The Attempt at a Solution



We know that -1 \leq \sin\frac{\pi}{x} \leq 1
\Leftrightarrow -\sqrt{x^3 + x^2} \leq \sqrt{x^3 + x^2}\sin\frac{\pi}{x} \leq \sqrt{x^3 + x^2} since \sqrt{y} \geq 0\forall y\in\mathbb{P}\cup\{0\}
Now, \lim_{x \to 0} -\sqrt{x^3 + x^2} = 0 = \lim_{x \to 0} \sqrt{x^3 + x^2}
\therefore \lim_{x \to 0} \sqrt{x^3 + x^2}\sin\frac{\pi}{x} = 0 by Sandwich Theorem.
 
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What's your question?
 
I want to know if it is correct.

What worries me is if I can state that -1 \leq \sin{\frac{\pi}{x}} \leq 1 since x is approaching 0, and right there \frac{\pi}{x} is undefined. Thanks in advance.
 
What you can say is -1 ≤ sin(θ) ≤ 1 for all real θ.

Therefore, -1 \leq \sin{\frac{\pi}{x}} \leq 1 provided that x ≠ 0 .

Since x approaches zero for your limit, x is not equal to zero, so you're fine.
 

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