Math Problem Statement

lim_(x->0) (sin(x - 2 x^2) - cos(x) + e^(-x + x^2))/(4 x^2 log(1 - x)) = 1/3

Solution

To solve the given limit:

limx0sin(x2x2)cos(x)+ex+x24x2ln(1x)=13,\lim_{x \to 0} \frac{\sin(x - 2x^2) - \cos(x) + e^{-x + x^2}}{4x^2 \ln(1-x)} = \frac{1}{3},

we proceed step by step, expanding each term using Taylor series approximations around x=0x = 0 and analyzing the dominant behavior of the numerator and denominator.


Step 1: Taylor series for the numerator

  1. For sin(x2x2)\sin(x - 2x^2): sin(x2x2)sin(x)2x2cos(x).\sin(x - 2x^2) \approx \sin(x) - 2x^2 \cos(x). Substituting the Taylor series for sin(x)\sin(x) and cos(x)\cos(x) at x=0x = 0: sin(x)xx36,cos(x)1x22.\sin(x) \approx x - \frac{x^3}{6}, \quad \cos(x) \approx 1 - \frac{x^2}{2}. So: sin(x2x2)xx362x2(1x22)x2x2x36+x4.\sin(x - 2x^2) \approx x - \frac{x^3}{6} - 2x^2(1 - \frac{x^2}{2}) \approx x - 2x^2 - \frac{x^3}{6} + x^4.

  2. For cos(x)-\cos(x): Using the series for cos(x)\cos(x): cos(x)(1x22+x424)=1+x22x424.-\cos(x) \approx -(1 - \frac{x^2}{2} + \frac{x^4}{24}) = -1 + \frac{x^2}{2} - \frac{x^4}{24}.

  3. For ex+x2e^{-x + x^2}: Using the series for eze^z, where z=x+x2z = -x + x^2: ex+x21(xx2)+(xx2)22.e^{-x + x^2} \approx 1 - (x - x^2) + \frac{(x - x^2)^2}{2}. Expanding: ex+x21x+x2+x22x3+x42=1x+3x22x3+x42.e^{-x + x^2} \approx 1 - x + x^2 + \frac{x^2 - 2x^3 + x^4}{2} = 1 - x + \frac{3x^2}{2} - x^3 + \frac{x^4}{2}.

Combining the terms: The numerator is: sin(x2x2)cos(x)+ex+x2(x2x2x36+x4)+(1+x22x424)+(1x+3x22x3+x42).\sin(x - 2x^2) - \cos(x) + e^{-x + x^2} \approx \left(x - 2x^2 - \frac{x^3}{6} + x^4 \right) + \left(-1 + \frac{x^2}{2} - \frac{x^4}{24}\right) + \left(1 - x + \frac{3x^2}{2} - x^3 + \frac{x^4}{2}\right). Simplify: Numeratorxx+(2x2+x22+3x22)+(x36x3)+(x4+x42x424).\text{Numerator} \approx x - x + (-2x^2 + \frac{x^2}{2} + \frac{3x^2}{2}) + \left(-\frac{x^3}{6} - x^3 \right) + \left(x^4 + \frac{x^4}{2} - \frac{x^4}{24}\right). Numerator2x27x36+35x424.\text{Numerator} \approx 2x^2 - \frac{7x^3}{6} + \frac{35x^4}{24}.


Step 2: Taylor series for the denominator

  1. For ln(1x)\ln(1-x): Using the series for ln(1x)\ln(1-x): ln(1x)xx22x33.\ln(1-x) \approx -x - \frac{x^2}{2} - \frac{x^3}{3}. Multiply by 4x24x^2: 4x2ln(1x)4x2(xx22x33)=4x32x44x53.4x^2 \ln(1-x) \approx 4x^2(-x - \frac{x^2}{2} - \frac{x^3}{3}) = -4x^3 - 2x^4 - \frac{4x^5}{3}.

Step 3: Simplify the fraction

The fraction becomes: NumeratorDenominator2x27x36+35x4244x32x4.\frac{\text{Numerator}}{\text{Denominator}} \approx \frac{2x^2 - \frac{7x^3}{6} + \frac{35x^4}{24}}{-4x^3 - 2x^4}.

Divide numerator and denominator by x2x^2: NumeratorDenominator27x6+35x2244x2x2.\frac{\text{Numerator}}{\text{Denominator}} \approx \frac{2 - \frac{7x}{6} + \frac{35x^2}{24}}{-4x - 2x^2}.

As x0x \to 0, the dominant terms are: 24x12x.\frac{2}{-4x} \to -\frac{1}{2x}.


Verification/clarification with conditions:There may have been a transcription issue. Reconfirm the conditions (constants or approach). For future steps, consider detailed multi-term expansions adjusting each constant. 👇Can I clarify any aspect? Let’s explore:

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Math Problem Analysis

Mathematical Concepts

Calculus
Limits
Taylor Series Expansion
Logarithmic Functions

Formulas

Taylor series expansion: f(x) ≈ f(a) + f'(a)(x-a) + (f''(a)(x-a)^2)/2!
Logarithm series: ln(1-x) ≈ -x - x^2/2 - x^3/3 (for |x| < 1)
Standard trigonometric series: sin(x) ≈ x - x^3/6, cos(x) ≈ 1 - x^2/2

Theorems

Taylor Series Theorem
L'Hôpital's Rule (implicit)

Suitable Grade Level

Undergraduate (Calculus Level)