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.
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 dxd[uv]=uv′+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) inside the difference quotient so that two separate quotients appear. The classic counterexample is x⋅x, which is x2 with derivative 2x, while the product of the derivatives would give 1. 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.
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.
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.
| Statement | Result | Comment |
|---|---|---|
| dxd[uv] | uv′+u′v | Two terms, always |
| dxd[uvw] | u′vw+uv′w+uvw′ | Three terms for three factors |
| dxd[cf] | cf′ | Not a product rule case: c is constant |
| dxd[x⋅x] | 2x | The counterexample to u′v′ |
The second row generalises the pattern: with n factors the answer has n terms, each one differentiating a single factor and copying the rest. Recognising that structure makes the three-factor case unnecessary to memorise.
Add and subtract the same quantity inside the difference quotient:
hu(x+h)v(x+h)−u(x)v(x)=hu(x+h)v(x+h)−u(x+h)v(x)+u(x+h)v(x)−u(x)v(x)
Grouping the first two terms and the last two terms produces u(x+h)⋅hv(x+h)−v(x)+v(x)⋅hu(x+h)−u(x). As h→0, u(x+h)→u(x) by continuity, the two quotients become v′ and u′, and the sum is uv′+u′v. The inserted quantity is the whole trick, and it is worth seeing once because the same device proves the quotient rule.
| Expression | Product rule needed? | Why |
|---|---|---|
| x2sinx | Yes | Two genuine factors in x |
| 3x2 | No | 3 is a constant; use the constant multiple rule |
| x(x+1) | Optional | Expanding to x2+x is faster |
| (x+1)5 | No | A composition; chain rule |
| f(x)g(x) with table values | Yes | The 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.
Borrowed idea: The product rule exists because the derivative of a product is not the product of the derivatives, and x⋅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 disguise | What you see on the page | What your brain must translate | Required execution |
|---|---|---|---|
| Named functions | P(x)=G(x)f(x) | Two factors, one known only by its derivative | P′=G′f+Gf′ |
| Table values | Four columns of numbers | The rule applied numerically | Substitute the four values, add |
| A rate model | v(t)=1.8t(1.25)−t | A linear factor times an exponential | Two terms, with the exponential copied twice |
| A triple product | x2sinxcosx | Three factors, three terms | Differentiate one factor per term |
① Differentiate x⋅x with the product rule and confirm that it gives 2x, matching the power rule. Then differentiate x2⋅x3 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 u, u′, 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.
Core Archetype (Simplest Form): Differentiate f(x)=x2ex.
Standard Execution:
- Name the factors. u=x2 and v=ex.
- Differentiate each alone. u′=2x and v′=ex.
- First term. uv′=x2ex.
- Second term. u′v=2xex.
- Assemble and factor. f′(x)=x2ex+2xex=xex(x+2).
Exam Variation (Context / Parameter Shift): The functions f and g are twice differentiable. The table gives f(4)=3, f′(4)=−2, g(4)=5, and g′(4)=6. Let k(x)=f(x)g(x). Find k′(4), and determine whether k is increasing or decreasing at x=4.
Solution: By the product rule, k′(4)=f(4)g′(4)+f′(4)g(4)=(3)(6)+(−2)(5)=18−10=8. Since k′(4)>0, the function k is increasing at x=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:
6D Cognitive Error Diagnosis (Where Your Thinking Broke):
MCQ 1
What is the derivative of f(x)=x3cosx?
MCQ 2
The table gives f(2)=4, f′(2)=−3, g(2)=5, and g′(2)=2. If h(x)=f(x)g(x), what is h′(2)?
MCQ 3
What is the derivative of y=xsinxcosx?
FRQ 4
The functions f and g are differentiable for all real x. Selected values of f, f′, g, and g′ are given in the table.
| x | f(x) | f′(x) | g(x) | g′(x) |
|---|---|---|---|---|
| 1 | 2 | −1 | 3 | 4 |
| 3 | −2 | 5 | 1 | 6 |
(a) Let h(x)=f(x)g(x). Find h′(1) and h′(3).
(b) Let k(x)=3f(x)g(x). Find k′(1) and explain how the constant multiple rule interacts with the product rule.
(c) Let m(x)=xf(x)g(x). Find m′(3).
(d) A classmate claims that 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.
| Rule | Example | |
|---|---|---|
| Two factors | uv′+u′v | x2ex→2xex+x2ex |
| Three factors | u′vw+uv′w+uvw′ | xsinxcosx→sinxcosx+xcos2x−xsin2x |
| Constant times product | c(uv′+u′v) | 3fg→3(fg′+f′g) |
| Sum inside a factor | Expand first if easier | x(x+1)→2x+1 |
| Composition | Chain rule | (x+1)5→5(x+1)4 |
Strengthen this topic with AP-style practice, or continue to next topic.
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 dxd[uv]=uv′+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) inside the difference quotient so that two separate quotients appear. The classic counterexample is x⋅x, which is x2 with derivative 2x, while the product of the derivatives would give 1. 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.
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.
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.
| Statement | Result | Comment |
|---|---|---|
| dxd[uv] | uv′+u′v | Two terms, always |
| dxd[uvw] | u′vw+uv′w+uvw′ | Three terms for three factors |
| dxd[cf] | cf′ | Not a product rule case: c is constant |
| dxd[x⋅x] | 2x | The counterexample to u′v′ |
The second row generalises the pattern: with n factors the answer has n terms, each one differentiating a single factor and copying the rest. Recognising that structure makes the three-factor case unnecessary to memorise.
Add and subtract the same quantity inside the difference quotient:
hu(x+h)v(x+h)−u(x)v(x)=hu(x+h)v(x+h)−u(x+h)v(x)+u(x+h)v(x)−u(x)v(x)
Grouping the first two terms and the last two terms produces u(x+h)⋅hv(x+h)−v(x)+v(x)⋅hu(x+h)−u(x). As h→0, u(x+h)→u(x) by continuity, the two quotients become v′ and u′, and the sum is uv′+u′v. The inserted quantity is the whole trick, and it is worth seeing once because the same device proves the quotient rule.
| Expression | Product rule needed? | Why |
|---|---|---|
| x2sinx | Yes | Two genuine factors in x |
| 3x2 | No | 3 is a constant; use the constant multiple rule |
| x(x+1) | Optional | Expanding to x2+x is faster |
| (x+1)5 | No | A composition; chain rule |
| f(x)g(x) with table values | Yes | The 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.
Borrowed idea: The product rule exists because the derivative of a product is not the product of the derivatives, and x⋅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 disguise | What you see on the page | What your brain must translate | Required execution |
|---|---|---|---|
| Named functions | P(x)=G(x)f(x) | Two factors, one known only by its derivative | P′=G′f+Gf′ |
| Table values | Four columns of numbers | The rule applied numerically | Substitute the four values, add |
| A rate model | v(t)=1.8t(1.25)−t | A linear factor times an exponential | Two terms, with the exponential copied twice |
| A triple product | x2sinxcosx | Three factors, three terms | Differentiate one factor per term |
① Differentiate x⋅x with the product rule and confirm that it gives 2x, matching the power rule. Then differentiate x2⋅x3 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 u, u′, 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.
Core Archetype (Simplest Form): Differentiate f(x)=x2ex.
Standard Execution:
- Name the factors. u=x2 and v=ex.
- Differentiate each alone. u′=2x and v′=ex.
- First term. uv′=x2ex.
- Second term. u′v=2xex.
- Assemble and factor. f′(x)=x2ex+2xex=xex(x+2).
Exam Variation (Context / Parameter Shift): The functions f and g are twice differentiable. The table gives f(4)=3, f′(4)=−2, g(4)=5, and g′(4)=6. Let k(x)=f(x)g(x). Find k′(4), and determine whether k is increasing or decreasing at x=4.
Solution: By the product rule, k′(4)=f(4)g′(4)+f′(4)g(4)=(3)(6)+(−2)(5)=18−10=8. Since k′(4)>0, the function k is increasing at x=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:
6D Cognitive Error Diagnosis (Where Your Thinking Broke):
MCQ 1
What is the derivative of f(x)=x3cosx?
MCQ 2
The table gives f(2)=4, f′(2)=−3, g(2)=5, and g′(2)=2. If h(x)=f(x)g(x), what is h′(2)?
MCQ 3
What is the derivative of y=xsinxcosx?
FRQ 4
The functions f and g are differentiable for all real x. Selected values of f, f′, g, and g′ are given in the table.
| x | f(x) | f′(x) | g(x) | g′(x) |
|---|---|---|---|---|
| 1 | 2 | −1 | 3 | 4 |
| 3 | −2 | 5 | 1 | 6 |
(a) Let h(x)=f(x)g(x). Find h′(1) and h′(3).
(b) Let k(x)=3f(x)g(x). Find k′(1) and explain how the constant multiple rule interacts with the product rule.
(c) Let m(x)=xf(x)g(x). Find m′(3).
(d) A classmate claims that 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.
| Rule | Example | |
|---|---|---|
| Two factors | uv′+u′v | x2ex→2xex+x2ex |
| Three factors | u′vw+uv′w+uvw′ | xsinxcosx→sinxcosx+xcos2x−xsin2x |
| Constant times product | c(uv′+u′v) | 3fg→3(fg′+f′g) |
| Sum inside a factor | Expand first if easier | x(x+1)→2x+1 |
| Composition | Chain rule | (x+1)5→5(x+1)4 |
Strengthen this topic with AP-style practice, or continue to next topic.