Unit 2The Product Rule
Concept 4.116Built for May 2027 AP Exam

The Product Rule

Reviewed by ACEpath AP Math Editorial Team

Last Updated: September 2026

30-second summary

A product does not differentiate factor by factor. The rule is ddx[uv]=uv′+u′v\frac{d}{dx}\bigl[u v\bigr] = u v' + u' v: each factor takes a turn being differentiated while the other is copied, and the two results are added. The proof is a subtraction trick, adding and subtracting u(x+h)v(x)u(x+h)v(x) inside the difference quotient so that two separate quotients appear. The classic counterexample is x⋅xx \cdot x, which is x2x^2 with derivative 2x2x, while the product of the derivatives would give 11. The exam uses this rule whenever a polynomial, trigonometric, or exponential factor meets another factor, and the most common failure is stopping after the first term.

On the AP Exam

The rule is scored through the derivative it produces, usually inside a tangent line, a rate, or a second derivative. Graders look for two terms, and a single-term answer is the signature failure. Because the two terms are added, order is free, but completeness is not.

What you’ll see
A function written as a product of two named functions, such as P(x)=G(x)f(x)P(x) = G(x)f(x), or an explicit product such as x2exx^2e^x or tcos⁡tt\cos t.
What you’ll do
Identify the two factors, differentiate each one alone, and assemble both terms before simplifying.
Memory hook
First times the derivative of the second, plus second times the derivative of the first. Say it out loud once per problem until it is automatic.
Use it on the exam
For MCQs: Count the terms in each option. The correct answer for a two-factor product almost always has two terms, and options with one term can be eliminated without any algebra. For FRQs: When a table supplies ff, gg, f′f', and g′g' at selected inputs, the product rule is being tested with numbers instead of formulas. Substitute the four table values directly rather than trying to reconstruct the functions.

Concept Review

Figure 1

Each factor gets one turn. The three chips show the rule on a polynomial partner, an exponential partner, and a factor known only through a table or a graph.

FIRST TIMES THE DERIVATIVE OF THE SECOND

The Rule, and the Shape of Its Answer

StatementResultComment
ddx[uv]\frac{d}{dx}\bigl[u v\bigr]uv′+u′vu v' + u' vTwo terms, always
ddx[uvw]\frac{d}{dx}\bigl[u v w\bigr]u′vw+uv′w+uvw′u'vw + uv'w + uvw'Three terms for three factors
ddx[cf]\frac{d}{dx}\bigl[c f\bigr]cf′c f'Not a product rule case: cc is constant
ddx[x⋅x]\frac{d}{dx}\bigl[x \cdot x\bigr]2x2xThe counterexample to u′v′u'v'

The second row generalises the pattern: with nn factors the answer has nn terms, each one differentiating a single factor and copying the rest. Recognising that structure makes the three-factor case unnecessary to memorise.

Where the Rule Comes From

Add and subtract the same quantity inside the difference quotient:

u(x+h)v(x+h)−u(x)v(x)h=u(x+h)v(x+h)−u(x+h)v(x)+u(x+h)v(x)−u(x)v(x)h\frac{u(x+h)v(x+h) - u(x)v(x)}{h} = \frac{u(x+h)v(x+h) - u(x+h)v(x) + u(x+h)v(x) - u(x)v(x)}{h}

Grouping the first two terms and the last two terms produces u(x+h)⋅v(x+h)−v(x)h+v(x)⋅u(x+h)−u(x)hu(x+h)\cdot\frac{v(x+h)-v(x)}{h} + v(x)\cdot\frac{u(x+h)-u(x)}{h}. As h→0h \to 0, u(x+h)→u(x)u(x+h) \to u(x) by continuity, the two quotients become v′v' and u′u', and the sum is uv′+u′vu v' + u' v. The inserted quantity is the whole trick, and it is worth seeing once because the same device proves the quotient rule.

When to Reach for It

ExpressionProduct rule needed?Why
x2sin⁡xx^2\sin xYesTwo genuine factors in xx
3x23x^2No33 is a constant; use the constant multiple rule
x(x+1)x(x+1)OptionalExpanding to x2+xx^2 + x is faster
(x+1)5(x+1)^5NoA composition; chain rule
f(x)g(x)f(x)g(x) with table valuesYesThe rule combines four known numbers

The third row matters on an exam: if expanding is cheaper, expand. The rule is a tool, not an obligation, and choosing the cheaper route is part of the skill.

Common Misconception Check

  • Myth 1: "The derivative of a product is the product of the derivatives." Truth: x⋅xx \cdot x gives 2x2x, not 11. The claim fails on the simplest possible example.
  • Myth 2: "One term is enough if the other factor is simple." Truth: A factor equal to xx still has a derivative of 11, and that term survives as a full term of the answer.
  • Myth 3: "The order of the two terms matters." Truth: They are added, so either order is correct. What matters is that both appear.

Topic-Specific Study Method

Two Terms Before Any Tidying

Borrowed idea: The product rule exists because the derivative of a product is not the product of the derivatives, and x⋅xx \cdot x is the counterexample that settles the question once and for all. Both factors change at once, so each factor gets a turn at being differentiated while the other is held still. Does the order matter? It does not. The two terms are added, so the discipline is to write both before simplifying either one, since students who tidy the first term immediately tend to forget that a second term is still owed.

Exam disguiseWhat you see on the pageWhat your brain must translateRequired execution
Named functionsP(x)=G(x)f(x)P(x) = G(x)f(x)Two factors, one known only by its derivativeP′=G′f+Gf′P' = G'f + Gf'
Table valuesFour columns of numbersThe rule applied numericallySubstitute the four values, add
A rate modelv(t)=1.8t(1.25)−tv(t) = 1.8t(1.25)^{-t}A linear factor times an exponentialTwo terms, with the exponential copied twice
A triple productx2sin⁡xcos⁡xx^2\sin x \cos xThree factors, three termsDifferentiate one factor per term

① Differentiate x⋅xx \cdot x with the product rule and confirm that it gives 2x2x, matching the power rule. Then differentiate x2⋅x3x^2 \cdot x^3 both ways: with the product rule and by combining first. The two answers agree, which is the best possible sanity check.

② On any table-based product question, write the four symbols uu, u′u', vv, v′v' on the page before substituting anything. Labelling first prevents the most common substitution error, which is putting a value where a derivative belongs.

③ Practise until the sentence is automatic: "first times the derivative of the second, plus second times the derivative of the first." Then practise the three-factor version by extending the pattern rather than memorising a new formula.

Training & Error Diagnosis

Core Archetype (Simplest Form): Differentiate f(x)=x2exf(x) = x^2 e^x.

Standard Execution:

  1. Name the factors. u=x2u = x^2 and v=exv = e^x.
  2. Differentiate each alone. u′=2xu' = 2x and v′=exv' = e^x.
  3. First term. uv′=x2exu v' = x^2 e^x.
  4. Second term. u′v=2xexu' v = 2x e^x.
  5. Assemble and factor. f′(x)=x2ex+2xex=xex(x+2)f'(x) = x^2e^x + 2xe^x = xe^x(x + 2).

Exam Variation (Context / Parameter Shift): The functions ff and gg are twice differentiable. The table gives f(4)=3f(4) = 3, f′(4)=−2f'(4) = -2, g(4)=5g(4) = 5, and g′(4)=6g'(4) = 6. Let k(x)=f(x)g(x)k(x) = f(x)g(x). Find k′(4)k'(4), and determine whether kk is increasing or decreasing at x=4x = 4.

Solution: By the product rule, k′(4)=f(4)g′(4)+f′(4)g(4)=(3)(6)+(−2)(5)=18−10=8k'(4) = f(4)g'(4) + f'(4)g(4) = (3)(6) + (-2)(5) = 18 - 10 = 8. Since k′(4)>0k'(4) > 0, the function kk is increasing at x=4x = 4. No formula for either function is needed, and none is available, which is exactly the point of a table-based version.

Common Traps & Failure Signals:

  1. The single term (Signal: an answer with one term for a two-factor product): the second turn of the rule never happened.
  2. The derivative swap (Signal: values substituted where derivatives belong): the table columns were read in the wrong order.
  3. The unnecessary rule (Signal: a constant handled as a second factor): the constant multiple rule would have been shorter and safer.

6D Cognitive Error Diagnosis (Where Your Thinking Broke):

  • Single-Factor Reflex →\rightarrow Write both terms before simplifying either.
  • Sign Leak: You lost the minus inside a cosine term.
  • Term Amnesia: You differentiated the first factor, wrote the answer, and never returned for the second term, leaving a result that is exactly half of the true derivative and therefore produces a plausible but wrong slope at every input.

Past 10-Year AP Exam Appearance

Testing Frequency
Most Years
The rule is scored through the derivative it produces, most often in a tangent line, a rate, or a table-based substitution
10-Year Total Count
5 of 9 Exams · 8 FRQ Parts
FRQ Years Appeared
2017 (Q6a, Q6b, Q6c), 2018 (Q5b), 2019 (Q6b), 2021 (Q4b), 2023 (Q1d, Q2b)

Practice Examples

MCQ 1

The Basic Shape

What is the derivative of f(x)=x3cos⁡xf(x) = x^3\cos x?

MCQ 2

Table Values

The table gives f(2)=4f(2) = 4, f′(2)=−3f'(2) = -3, g(2)=5g(2) = 5, and g′(2)=2g'(2) = 2. If h(x)=f(x)g(x)h(x) = f(x)g(x), what is h′(2)h'(2)?

MCQ 3

The Triple Product

What is the derivative of y=xsin⁡xcos⁡xy = x\sin x \cos x?

FRQ 4

A Table-Based Product

The functions ff and gg are differentiable for all real xx. Selected values of ff, f′f', gg, and g′g' are given in the table.

xxf(x)f(x)f′(x)f'(x)g(x)g(x)g′(x)g'(x)
1122−1-13344
33−2-2551166

(a) Let h(x)=f(x)g(x)h(x) = f(x)g(x). Find h′(1)h'(1) and h′(3)h'(3).

(b) Let k(x)=3f(x)g(x)k(x) = 3f(x)g(x). Find k′(1)k'(1) and explain how the constant multiple rule interacts with the product rule.

(c) Let m(x)=xf(x)g(x)m(x) = x f(x) g(x). Find m′(3)m'(3).

(d) A classmate claims that h′(3)=f′(3)g′(3)h'(3) = f'(3)g'(3). Use the table to show that the claim is false, and explain which structural feature of the product rule the claim ignores.

Cheatsheet

The Product Rule

RuleExample
Two factorsuv′+u′vuv' + u'vx2ex→2xex+x2exx^2e^x \to 2xe^x + x^2e^x
Three factorsu′vw+uv′w+uvw′u'vw + uv'w + uvw'xsin⁡xcos⁡x→sin⁡xcos⁡x+xcos⁡2x−xsin⁡2xx\sin x\cos x \to \sin x\cos x + x\cos^2x - x\sin^2x
Constant times productc(uv′+u′v)c(uv' + u'v)3fg→3(fg′+f′g)3fg \to 3(fg' + f'g)
Sum inside a factorExpand first if easierx(x+1)→2x+1x(x+1) \to 2x+1
CompositionChain rule(x+1)5→5(x+1)4(x+1)^5 \to 5(x+1)^4
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