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Squeeze theorem applied to |x|^(3/2) sin(1/x) as x → 0

Consider the function f(x)=∣x∣3/2sin⁡(1x)f(x) = |x|^{3/2}\sin\left(\dfrac{1}{x}\right), defined for x≠0x \neq 0.

Use the Squeeze Theorem to evaluate lim⁡x→0∣x∣3/2sin⁡(1x).\lim_{x \to 0} |x|^{3/2}\sin\left(\frac{1}{x}\right). Find bounding functions g(x)g(x) and h(x)h(x) with g(x)≤f(x)≤h(x)g(x) \le f(x) \le h(x) near x=0x=0, show that g(x)g(x) and h(x)h(x) have the same limit at 00, and state the value of the limit.

Hint

Recall that sin⁡(1/x)\sin(1/x) oscillates but always satisfies −1≤sin⁡(1/x)≤1-1 \le \sin(1/x) \le 1 for x≠0x \neq 0. Multiply this inequality through by ∣x∣3/2|x|^{3/2}, which is nonnegative, to get bounds on f(x)f(x), then take the limit of the outer bounds.

Step-by-step solution

Since −1≤sin⁡(1/x)≤1-1 \le \sin(1/x) \le 1 for all x≠0x \neq 0, and ∣x∣3/2≥0|x|^{3/2} \ge 0, multiplying the inequality by ∣x∣3/2|x|^{3/2} preserves the direction of the inequalities: −∣x∣3/2≤∣x∣3/2sin⁡(1x)≤∣x∣3/2.-|x|^{3/2} \le |x|^{3/2}\sin\left(\frac{1}{x}\right) \le |x|^{3/2}. This gives us the squeeze bounds g(x)=−∣x∣3/2g(x) = -|x|^{3/2} and h(x)=∣x∣3/2h(x) = |x|^{3/2}.

Now evaluate the limits of the bounding functions: lim⁡x→0∣x∣3/2=0\lim_{x\to 0} |x|^{3/2} = 0 and lim⁡x→0−∣x∣3/2=0\lim_{x\to 0} -|x|^{3/2} = 0, since raising ∣x∣|x| to a positive power sends it to 00 as x→0x \to 0. Since both bounds tend to 00, the Squeeze Theorem guarantees that f(x)f(x) is trapped between two quantities both approaching 00, so lim⁡x→0∣x∣3/2sin⁡(1x)=0.\lim_{x \to 0} |x|^{3/2}\sin\left(\frac{1}{x}\right) = 0.

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