Determining limits using algebraic properties of limits

AP Calculus BC· AP Calculus BC CED — Limits and Continuity· 14 min read

1. Basic Limit Laws and Direct Substitution★☆☆☆☆⏱ 4 min

The fundamental algebraic properties of limits, called limit laws, let us break complex limits into simpler solvable parts. All laws assume that $lim_{x \to a} f (x) = L$ and $lim_{x \to a} g (x) = M$ both exist as finite real numbers.

Constant multiple: $lim_{x \to a} c f (x) = c L$ for any constant $c$
Sum/difference: $lim_{x \to a} [f (x) \pm g (x)] = L \pm M$
Product: $lim_{x \to a} [f (x) g (x)] = L M$
Quotient: $lim_{x \to a} \frac{f ( x )}{g ( x )} = \frac{L}{M}$ , if and only if $M \neq = 0$
Power/root: $lim_{x \to a} [f (x)]^{n} = L^{n}$ and $lim_{x \to a} n f (x) = n L$ (for even $n$ , $L \geq 0$ )

📘 Definition

Direct Substitution

For any function continuous at $x = a$ , $lim_{x \to a} f (x) = f (a)$ . This works because continuity is defined as the limit equaling the function value, and applies to all standard functions on their domains.

📐 Worked Example

Evaluate $lim_{x \to 2} (3 x^{2} - 5 x + 2 x + 1)$ using algebraic limit properties.

1
First, confirm the function is defined at $x = 2$ : the expression inside the square root is $2 (2) + 1 = 5 > 0$ , so $x = 2$ is in the domain, and direct substitution applies.
2
Split the limit using sum/difference and constant multiple rules:
3
$\lim_{x \to 2} \left(3x^2 - 5x + \sqrt{2x + 1}\right) = 3\left(\lim_{x \to 2} x\right)^2 - 5\left(\lim_{x \to 2} x\right) + \sqrt{\lim_{x \to 2} (2x + 1)}$
4
Use the basic identity $lim_{x \to 2} x = 2$ and substitute values:
5
$= 3(2)^2 - 5(2) + \sqrt{2(2) + 1} = 12 - 10 + \sqrt{5} = 2 + \sqrt{5}$
6
The final limit is $2 + 5$ .

Exam tip:

Always confirm the evaluation point is in the function's domain before using direct substitution—if it is, no extra work is needed.

2. Factoring and Canceling for 0/0 Indeterminate Forms★★☆☆☆⏱ 3 min

When you substitute $x = a$ into a rational function and get $\frac{0}{0}$ , you have an indeterminate form. This does not mean the limit does not exist—it only means the quotient law cannot be applied directly. $\frac{0}{0}$ almost always means the numerator and denominator share a common factor of $(x - a)$ . Because we take the limit as $x \to a$ , $x$ never actually equals $a$ , so we can safely cancel the common factor and use direct substitution on the simplified expression.

📐 Worked Example

Evaluate $lim_{x \to - 3} \frac{x ^{2} + x - 6}{x ^{2} - 9}$ .

1
First test direct substitution: at $x = - 3$ , numerator is $(- 3)^{2} + (- 3) - 6 = 0$ , denominator is $(- 3)^{2} - 9 = 0$ , so we have a 0/0 indeterminate form, so factoring is required.
2
Factor both the numerator and denominator:
3
$x^2 + x - 6 = (x + 3)(x - 2) \quad \text{and} \quad x^2 - 9 = (x + 3)(x - 3)$
4
Cancel the common factor $(x + 3)$ , which is valid because $x \to - 3$ so $x \neq = - 3$ and $(x + 3) \neq = 0$ . This simplifies the limit to:
5
$\lim_{x \to -3} \frac{x - 2}{x - 3}$
6
Use direct substitution on the simplified expression:
7
$\frac{-3 - 2}{-3 - 3} = \frac{-5}{-6} = \frac{5}{6}$
8
The limit equals $\frac{5}{6}$ .

Exam tip:

If you get 0/0 after substitution, always look for a common linear factor first—9 times out of 10 on the AP exam, this factor cancels cleanly.

3. Rationalizing for Radical Indeterminate Forms★★★☆☆⏱ 3 min

When 0/0 indeterminate forms include radicals, factoring will not work directly, so we use the method of rationalizing. This relies on the difference of squares identity: $(a - b) (a + b) = a^{2} - b^{2}$ . We multiply both numerator and denominator by the conjugate of the radical expression (the conjugate changes the sign between the radical term and the constant term, not inside the radical) to eliminate the radical, reveal a common factor, then cancel and substitute.

📐 Worked Example

Evaluate $lim_{x \to 0} \frac{x + 4 - 2}{x}$ .

1
Test direct substitution: $\frac{0 + 4 - 2}{0} = \frac{0}{0}$ , which is indeterminate. Factoring is not possible here due to the radical, so we use rationalizing.
2
Multiply numerator and denominator by the conjugate of the numerator, $x + 4 + 2$ :
3
$\lim_{x \to 0} \frac{(\sqrt{x + 4} - 2)(\sqrt{x + 4} + 2)}{x(\sqrt{x + 4} + 2)}$
4
Multiply out the numerator using difference of squares:
5
$(x + 4) - (2)^2 = x + 4 - 4 = x$
6
This simplifies the expression to:
7
$\lim_{x \to 0} \frac{x}{x(\sqrt{x + 4} + 2)}$
8
Cancel the common $x$ factor (valid because $x \to 0$ , so $x \neq = 0$ ), leaving:
9
$\lim_{x \to 0} \frac{1}{\sqrt{x + 4} + 2}$
10
Direct substitute to get the final result:
11
$\frac{1}{\sqrt{4} + 2} = \frac{1}{4}$
12
The limit equals $\frac{1}{4}$ .

Exam tip:

Always place the conjugate on the side that contains the radical—if the radical is in the numerator, multiply by the numerator's conjugate; if it is in the denominator, use the denominator's conjugate.

4. Algebraic Evaluation of Limits for Piecewise Functions★★★☆☆⏱ 4 min

To find the limit as $x$ approaches a point where a piecewise function changes its rule, you evaluate the left-hand limit ( $x \to a^{-}$ ) using the rule that applies for $x < a$ , and the right-hand limit ( $x \to a^{+}$ ) using the rule that applies for $x > a$ . A two-sided limit exists if and only if both one-sided limits are equal. You use the same algebraic properties (direct substitution, factoring, rationalizing) to evaluate each one-sided limit separately.

📐 Worked Example

Let $f (x) = {\frac{x ^{2} - 4}{x - 2} a x + 6 x < 2 x \geq 2$ . Find the value of $a$ such that $lim_{x \to 2} f (x)$ exists.

1
Evaluate the left-hand limit ( $x \to 2^{-}$ ), so use the first rule for $x < 2$ : $lim_{x \to 2^{-}} \frac{x ^{2} - 4}{x - 2}$ . Direct substitution gives 0/0, so factor.
2
$\frac{(x - 2)(x + 2)}{x - 2} = x + 2$
3
After canceling $(x - 2)$ (valid for $x \neq = 2$ ), the left-hand limit is $2 + 2 = 4$ .
4
Evaluate the right-hand limit ( $x \to 2^{+}$ ), so use the second rule for $x \geq 2$ : $lim_{x \to 2^{+}} a x + 6 = 2 a + 6$ by direct substitution.
5
Set left-hand limit equal to right-hand limit for the two-sided limit to exist: $4 = 2 a + 6$ . Solve for $a$ : $2 a = - 2$ , so $a = - 1$ .

✓ Quick check

Test your understanding with this AP-style multiple choice question:

Which of the following is equal to $lim_{x \to 4} \frac{2 x - 8}{x ^{2} - 3 x - 4}$ ?
- $\frac{2}{5}$
- $\frac{1}{2}$
- $0$
- The limit does not exist

Exam tip:

Always double-check which piece corresponds to which side: $a^{-}$ means $x < a$ , so use the rule for $x < a$ , not the reverse.

5. Common Pitfalls

Wrong move:

Concluding a limit does not exist immediately after getting 0/0 from direct substitution.

Why:

Students confuse the function being undefined at $a$ with the limit not existing at $a$ . 0/0 is an indeterminate form, not a final conclusion.

Correct move:

If you get 0/0, always proceed to factoring or rationalizing to simplify the expression before concluding the limit does not exist.

Wrong move:

Canceling the $(x - a)$ factor and then concluding $f (a)$ equals the simplified value.

Why:

Students confuse the limit as $x \to a$ with the value of the function at $a$ .

Correct move:

Explicitly note that $x \neq = a$ when canceling, so the equality only holds for the limit, not the function value at $a$ .

Wrong move:

Using the quotient law when the denominator limit is zero, without checking the numerator limit.

Why:

Students forget that 0/0 is indeterminate, while non-zero/zero has no finite limit.

Correct move:

If the denominator limit is zero, check the numerator first: if it is also zero, simplify; if not, the limit does not exist (or is infinite).

Wrong move:

Using the wrong conjugate by changing the sign inside the radical instead of between the terms.

Why:

Students confuse the position of the sign change in the conjugate.

Correct move:

Remember the conjugate of $A - B$ is $A + B$ : only flip the sign between the two terms, leave the radical term unchanged.

Wrong move:

Evaluating the wrong one-sided limit for piecewise functions, using the $x \geq a$ rule for $x \to a^{-}$ .

Why:

Students mix up the notation for left and right limits.

Correct move:

Write a quick reminder: $a^{-}$ = less than $a$ , $a^{+}$ = greater than $a$ , then match to the correct piece rule.

6. Quick Reference Cheatsheet

Category	Rule/Formula	Notes
Basic Constant/Identity	$lim_{x \to a} c = c$ , $lim_{x \to a} x = a$	Applies for all constants $c$ and all real $a$
Sum/Difference Rule	$lim_{x \to a} [f (x) \pm g (x)] = lim f (x) \pm lim g (x)$	Requires both limits exist and are finite
Constant Multiple Rule	$lim_{x \to a} [c f (x)] = c lim_{x \to a} f (x)$	Requires the limit of $f (x)$ exists
Product/Quotient Rule	$lim [f g] = (lim f) (lim g)$ , $lim \frac{f}{g} = \frac{l i m f}{l i m g}$	Quotient rule only applies if $lim g \neq = 0$
Power/Root Rule	$lim [f (x)]^{n} = (lim f (x))^{n}$ , $lim n f (x) = n lim f (x)$	For even $n$ , requires $lim f (x) \geq 0$
Direct Substitution	If $f$ is continuous at $a$ , $lim_{x \to a} f (x) = f (a)$	Works for all continuous functions on their domains
0/0 Indeterminate Form	Simplify via factoring or rationalizing, cancel $(x - a)$ , substitute	Valid because $x \to a$ means $x \neq = a$
Two-Sided Limit Existence	$lim_{x \to a} f (x) = L ⟺ lim_{x \to a^{-}} f (x) = lim_{x \to a^{+}} f (x) = L$	Required for all piecewise function limit problems

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Evaluate limit of rational function
2022 · FRQ
Find constant for existing limit

What's Next

This topic is the foundational algebraic tool for every subsequent limit-based topic in AP Calculus BC. Next, you will apply these properties to evaluate infinite limits and limits at infinity to find asymptotes, and later test for convergence of series. Mastery here is required for core topics like finding derivatives via the limit definition and evaluating improper integrals, and builds intuition for indeterminate forms needed for L'Hospital's Rule later in the course.