Limit Evaluation Techniques
Basic Limit Laws
lim[x→a] [f(x) ± g(x)] = lim[x→a] f(x) ± lim[x→a] g(x)
lim[x→a] [f(x) · g(x)] = lim[x→a] f(x) · lim[x→a] g(x)
lim[x→a] [f(x) / g(x)] = lim[x→a] f(x) / lim[x→a] g(x) (if lim g(x) ≠ 0)
Indeterminate Forms
- 0/0: Use L'Hôpital's Rule, factoring, or rationalization
- ∞/∞: Use L'Hôpital's Rule or divide by highest power
- 0·∞: Rewrite as a fraction to get 0/0 or ∞/∞
- ∞-∞: Factor out common terms or rationalize
- 1^∞, 0^0, ∞^0: Use logarithms and exponentials
L'Hôpital's Rule
If lim[x→a] f(x)/g(x) = 0/0 or ∞/∞, then:
lim[x→a] f(x)/g(x) = lim[x→a] f'(x)/g'(x)
Important Limits
Trigonometric
lim[x→0] sin(x)/x = 1
lim[x→0] (1-cos(x))/x = 0
Exponential
lim[x→∞] (1+1/x)^x = e
lim[x→0] (e^x-1)/x = 1
Logarithmic
lim[x→0+] ln(x)/x = -∞
lim[x→0+] x·ln(x) = 0
Worked Examples
Example 1: Product Rule
Problem: Find the derivative of f(x) = x²·sin(x)
Solution:
• Using the product rule: (uv)' = u'v + uv'
• Let u = x² and v = sin(x)
• u' = 2x and v' = cos(x)
• f'(x) = (2x)·sin(x) + x²·cos(x)
• f'(x) = 2x·sin(x) + x²·cos(x)
Answer: f'(x) = 2x·sin(x) + x²·cos(x)
Example 2: Chain Rule
Problem: Find the derivative of f(x) = sin(3x² + 1)
Solution:
• Using the chain rule: [f(g(x))]' = f'(g(x))·g'(x)
• Outer function: f(u) = sin(u), f'(u) = cos(u)
• Inner function: g(x) = 3x² + 1, g'(x) = 6x
• f'(x) = cos(3x² + 1)·6x
• f'(x) = 6x·cos(3x² + 1)
Answer: f'(x) = 6x·cos(3x² + 1)
Example 3: Integration by Substitution
Problem: Evaluate ∫ 2x·√(x² + 1) dx
Solution:
• Let u = x² + 1, then du = 2x dx
• The integral becomes ∫ √u du
• ∫ u^(1/2) du = (2/3)u^(3/2) + C
• Substitute back: (2/3)(x² + 1)^(3/2) + C
Answer: (2/3)(x² + 1)^(3/2) + C
Example 4: L'Hôpital's Rule
Problem: Evaluate lim[x→0] (sin(x) - x)/x³
Solution:
• Direct substitution gives 0/0 (indeterminate)
• Apply L'Hôpital's rule: differentiate numerator and denominator
• lim[x→0] (cos(x) - 1)/(3x²) = 0/0 (still indeterminate)
• Apply L'Hôpital's rule again: lim[x→0] (-sin(x))/(6x) = 0/0
• Apply once more: lim[x→0] (-cos(x))/6 = -1/6
Answer: -1/6