Math Problem Statement

evaluate the limit as x approaches 0 of (cosx)^(3/x^2)

Solution

We are tasked with evaluating the following limit:

limx0(cos(x))3x2.\lim_{x \to 0} \left( \cos(x) \right)^{\frac{3}{x^2}}.

Step-by-step approach:

  1. Take the natural logarithm of the expression:
    To simplify this type of limit, we start by taking the natural logarithm of the expression:

    y=(cos(x))3x2    ln(y)=3x2ln(cos(x)).y = \left( \cos(x) \right)^{\frac{3}{x^2}} \implies \ln(y) = \frac{3}{x^2} \ln(\cos(x)).

    Now, we need to evaluate the limit of ln(y)\ln(y) as x0x \to 0.

  2. Evaluate the limit of ln(cos(x))\ln(\cos(x)):
    As x0x \to 0, cos(x)1\cos(x) \to 1, and ln(cos(x))\ln(\cos(x)) approaches ln(1)=0\ln(1) = 0. To get a more precise approximation near 0, we use the Taylor series expansion of cos(x)\cos(x) around x=0x = 0:

    cos(x)1x22+O(x4).\cos(x) \approx 1 - \frac{x^2}{2} + O(x^4).

    Using this, we approximate ln(cos(x))\ln(\cos(x)) for small xx:

    ln(cos(x))ln(1x22).\ln(\cos(x)) \approx \ln\left(1 - \frac{x^2}{2}\right).

    For small xx, we can use the approximation ln(1+z)z\ln(1 + z) \approx z when zz is small. Therefore:

    ln(cos(x))x22.\ln(\cos(x)) \approx -\frac{x^2}{2}.

  3. Substitute this into the expression for ln(y)\ln(y):
    Now substitute this approximation into the expression for ln(y)\ln(y):

    ln(y)=3x2(x22)=32.\ln(y) = \frac{3}{x^2} \left( -\frac{x^2}{2} \right) = -\frac{3}{2}.

  4. Exponentiate to find yy:
    To find yy, we exponentiate both sides:

    y=e32.y = e^{-\frac{3}{2}}.

Thus, the value of the original limit is:

limx0(cos(x))3x2=e32.\lim_{x \to 0} \left( \cos(x) \right)^{\frac{3}{x^2}} = e^{-\frac{3}{2}}.

Final Answer:

e32.\boxed{e^{-\frac{3}{2}}}.

Would you like more details on any step, or do you have any questions about this approach?

Here are some related questions you might find interesting:

  1. How can we compute the limit of (sin(x))3x2\left( \sin(x) \right)^{\frac{3}{x^2}} as x0x \to 0?
  2. What is the Taylor series expansion of cos(x)\cos(x) near x=0x = 0?
  3. How do you handle limits involving indeterminate forms like 00^\infty?
  4. What are some other techniques for solving limits of exponential functions?
  5. How does L'Hopital's rule apply to this kind of limit?

Tip: When dealing with limits of the form f(x)g(x)f(x)^{g(x)}, taking the logarithm of both sides often simplifies the process, especially when f(x)f(x) approaches 1 and g(x)g(x) grows large.

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Math Problem Analysis

Mathematical Concepts

Limits
Exponential Functions
Logarithms
Taylor Series

Formulas

ln(a^b) = b*ln(a)
cos(x) ≈ 1 - x^2/2 for small x
lim x->0 (cos(x))^n = e^(-n/2)

Theorems

Limit of exponential functions
Taylor Series Expansion

Suitable Grade Level

Grades 11-12