Math Problem Statement

Evaluate the limit: lim(x → π/2) [ln(csc x) / (x - π/2)^2].

Solution

This limit problem involves evaluating:

limxπ/2ln(cscx)(xπ/2)2.\lim_{x \to \pi/2} \frac{\ln(\csc x)}{(x - \pi/2)^2}.

Solution Steps:

  1. Expand ln(cscx)\ln(\csc x): Recall cscx=1sinx\csc x = \frac{1}{\sin x}, so: ln(cscx)=ln(1sinx)=ln(sinx).\ln(\csc x) = \ln\left(\frac{1}{\sin x}\right) = -\ln(\sin x). Thus, the expression becomes: limxπ/2ln(sinx)(xπ/2)2.\lim_{x \to \pi/2} \frac{-\ln(\sin x)}{(x - \pi/2)^2}.

  2. Behavior of sinx\sin x as xπ/2x \to \pi/2: Near x=π/2x = \pi/2, sinx112(xπ/2)2\sin x \approx 1 - \frac{1}{2}(x - \pi/2)^2 by Taylor expansion. Substituting this approximation: ln(sinx)ln(112(xπ/2)2).\ln(\sin x) \approx \ln\left(1 - \frac{1}{2}(x - \pi/2)^2\right).

  3. Simplify ln(1u)\ln(1 - u): Using ln(1u)u\ln(1 - u) \approx -u for small uu, we have: ln(112(xπ/2)2)12(xπ/2)2.\ln\left(1 - \frac{1}{2}(x - \pi/2)^2\right) \approx -\frac{1}{2}(x - \pi/2)^2.

  4. Substitute back: The numerator ln(sinx)-\ln(\sin x) becomes: (12(xπ/2)2)=12(xπ/2)2.-\left(-\frac{1}{2}(x - \pi/2)^2\right) = \frac{1}{2}(x - \pi/2)^2.

  5. Simplify the limit: The expression is now: limxπ/212(xπ/2)2(xπ/2)2.\lim_{x \to \pi/2} \frac{\frac{1}{2}(x - \pi/2)^2}{(x - \pi/2)^2}. Cancel (xπ/2)2(x - \pi/2)^2 from the numerator and denominator: limxπ/212=12.\lim_{x \to \pi/2} \frac{1}{2} = \frac{1}{2}.

Final Answer:

12\boxed{\frac{1}{2}}

Let me know if you'd like further clarification or a breakdown of any step.


Related Questions:

  1. What is the Taylor expansion of sinx\sin x near x=π/2x = \pi/2?
  2. Why does ln(1u)u\ln(1 - u) \approx -u for small uu?
  3. How do we handle indeterminate forms in limits like this one?
  4. Can this limit be evaluated using L'Hôpital's Rule?
  5. What are the implications of Taylor expansions in evaluating limits?

Tip: Always simplify expressions by approximating small terms when approaching indeterminate forms in limits.

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Math Problem Analysis

Mathematical Concepts

Limits
Taylor Expansion
Logarithmic Properties

Formulas

ln(csc x) = -ln(sin x)
Taylor series approximation: sin x ≈ 1 - (1/2)(x - π/2)^2 near x = π/2
Logarithmic approximation: ln(1 - u) ≈ -u for small u

Theorems

Taylor Series
Properties of Logarithms
L'Hôpital's Rule (optional)

Suitable Grade Level

Undergraduate (Calculus I/II)