Math Problem Statement

Evaluate the limit \(\lim_{x \to 0} \frac{|\sin x|}{\sqrt{1 - \cos x}}\).

Solution

To evaluate the given limit:

limx0sinx1cosx\lim_{x \to 0} \frac{|\sin x|}{\sqrt{1 - \cos x}}

it is noted that the modulus on sinx|\sin x| can be simplified as sinx\sin x itself because xx approaches 00 and sinx\sin x is positive in the vicinity of x=0x = 0.

This reduces the expression to:

limx0sinx1cosx\lim_{x \to 0} \frac{\sin x}{\sqrt{1 - \cos x}}

Now, let's simplify further. We use the trigonometric identity 1cosx=2sin2x21 - \cos x = 2 \sin^2 \frac{x}{2}. Substituting this identity into the denominator:

1cosx=2sin2x2=2sinx2\sqrt{1 - \cos x} = \sqrt{2 \sin^2 \frac{x}{2}} = \sqrt{2} |\sin \frac{x}{2}|

Since x0x \to 0, sinx2\sin \frac{x}{2} is also positive. Thus, sinx2=sinx2|\sin \frac{x}{2}| = \sin \frac{x}{2}. The limit becomes:

limx0sinx2sinx2\lim_{x \to 0} \frac{\sin x}{\sqrt{2} \sin \frac{x}{2}}

Using the double-angle identity sinx=2sinx2cosx2\sin x = 2 \sin \frac{x}{2} \cos \frac{x}{2}, substitute for sinx\sin x:

limx02sinx2cosx22sinx2\lim_{x \to 0} \frac{2 \sin \frac{x}{2} \cos \frac{x}{2}}{\sqrt{2} \sin \frac{x}{2}}

Cancel sinx2\sin \frac{x}{2} in the numerator and denominator (it is non-zero for small xx):

limx02cosx22\lim_{x \to 0} \frac{2 \cos \frac{x}{2}}{\sqrt{2}}

As x0x \to 0, cosx2cos0=1\cos \frac{x}{2} \to \cos 0 = 1. Thus, the limit evaluates to:

22=2\frac{2}{\sqrt{2}} = \sqrt{2}

Final Answer:

limx0sinx1cosx=2\lim_{x \to 0} \frac{|\sin x|}{\sqrt{1 - \cos x}} = \sqrt{2}

Let me know if you have further questions, or if you want additional details on any step.


Here are some related questions for exploration:

  1. What happens if the limit was approaching 0-0 instead of 00?
  2. How is the modulus affecting the behavior of sinx\sin x in different quadrants?
  3. What other trigonometric identities might simplify similar expressions?
  4. How can L’Hôpital’s Rule be applied to verify this result?
  5. What happens if the denominator includes 1+cosx1 + \cos x instead of 1cosx1 - \cos x?

Tip: Always explore trigonometric identities when dealing with limits involving sine and cosine near x=0x = 0.

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Math Problem Analysis

Mathematical Concepts

Limits
Trigonometry
Simplification using identities

Formulas

\(1 - \cos x = 2 \sin^2 \frac{x}{2}\)
\(\sin x = 2 \sin \frac{x}{2} \cos \frac{x}{2}\)

Theorems

Limit theorems
Trigonometric identities

Suitable Grade Level

Grades 11-12