Two Factors, One Motion
Figure 1 — The chain rule produces two factors, not one. The outside function is differentiated at the untouched inside, and the derivative of the inside arrives as a multiplicand.
Reviewed by ACEpath AP Math Editorial Team
Last Updated: September 2026
30-second summary
The chain rule is what keeps every other derivative rule alive inside a composite function. Differentiate the outside, leave the inside exactly as it stood, then multiply by the derivative of the inside. For f(g(x)) the result is f′(g(x))⋅g′(x), and that second factor is a multiplication, never an addition. The rule also stacks, because a function nested three deep hands you three factors in a chain. Almost every lost point on this topic is a second factor that never got written down, not a rule that was known incorrectly. The fix is procedural rather than conceptual: name the inside before you differentiate anything, then write the second factor before simplifying the first.
The chain rule is rarely a question by itself. It is the step that makes somebody else's question possible, and it sits inside the first line of tangent-line, rate-of-change, and implicit work. A missing factor there does not stay local; it changes every number that follows it.
Figure 1 — The chain rule produces two factors, not one. The outside function is differentiated at the untouched inside, and the derivative of the inside arrives as a multiplicand.
The only decision the rule requires is which layer is outside. Everything after that is bookkeeping.
| Function | Outside f | Inside g | Derivative |
|---|---|---|---|
| sin3x | sinu | 3x | 3cos3x |
| ex2 | eu | x2 | 2xex2 |
| (2x+1)5 | u5 | 2x+1 | 10(2x+1)4 |
| ln(x2+1) | lnu | x2+1 | x2+12x |
| 4x−3 | u1/2 | 4x−3 | 4x−32 |
The outside is the function whose argument is the whole inside expression. In sin2x the outside is the squaring and the inside is sinx; in 2sinx the outside is the exponential and the inside is sinx. Reading the expression aloud with the phrase "of" inserted at the right place settles the question in one pass.
Borrowed idea: The chain rule is one sentence, and it applies to every nested function you will meet. Differentiate the outside first, and leave the inside expression exactly as it was. Then multiply that result by the derivative of the inside, which is the second factor. Those two factors never merge into one, and neither of them may be dropped. Nearly every lost point in this topic is a second factor that never got written down. The rule stacks cleanly, because every additional layer contributes one more factor to the product.
| Exam disguise | What you see on the page | What your brain must translate | Required execution |
|---|---|---|---|
| A linear inside | sin5x, e3x, 2x+1 | The argument carries a constant slope | Multiply by that constant, even when it is 1 |
| A power inside | sin(x2), ex2 | The inside is not linear | Multiply by 2x, not by 2 |
| A function inside | sin(cosx), esinx | Two layers, so two factors | Differentiate outward, multiplying at each layer |
| A power of a function | sin2x, (x2+1)3 | The exponent is outside, the function is inside | Bring the power down, keep the inside, then multiply |
① Write f(x)=sin(3x) and differentiate it three times in a row, checking that a factor of 3 appears each time. The rule is the same motion repeated, and the repetition is what makes it stick.
② Take five expressions from your own homework that contain nested parentheses, and for each one write only the inside function, without differentiating. Naming the inside is the whole decision, and it takes about thirty seconds per expression.
③ On any tangent-line question, differentiate completely first and substitute second. Substituting into a half-differentiated expression silently discards the second factor, and the number you get afterwards looks entirely plausible.
Core Archetype (Simplest Form): Find dxd[sin(3x)+ex2].
Standard Execution:
- Split. The sum rule lets each term be handled alone.
- First term. The outside is sine, the inside is 3x, so the derivative is 3cos3x.
- Second term. The outside is the exponential, the inside is x2, so the derivative is 2xex2.
- Assemble. The derivative is 3cos3x+2xex2.
- Stop. No simplification is possible, and none is required.
Exam Variation (Context / Parameter Shift): Let f(x)=(2x+1)5. Find the equation of the line tangent to the graph of f at x=1.
Solution: The chain rule gives f′(x)=5(2x+1)4⋅2=10(2x+1)4. At x=1 the point is (1,243) and the slope is f′(1)=10⋅81=810. The tangent line is y−243=810(x−1). The factor of 2 inside the power is the entire difference between 810 and the 405 that a missing chain rule would produce.
Common Traps & Failure Signals:
6D Cognitive Error Diagnosis (Where Your Thinking Broke):
MCQ 1
If f(x)=sin(3x), what is f′(x)?
MCQ 2
Let g(x)=(2x+1)5. What is g′(0)?
MCQ 3
What is the derivative of h(x)=ex2?
FRQ 4
Let f be the function defined by f(x)=3x+1 for x≥−31.
(a) Find f′(x).
(b) Find the equation of the line tangent to the graph of f at x=1.
(c) The function g is defined by g(x)=f(x2). Find g′(x) and evaluate g′(1).
(d) A classmate writes f′(x)=23x+11 and claims the answer is complete. Identify the missing piece and explain what it changes about the slope at x=1.
| Move | |
|---|---|
| Argument is a bare x | No chain rule needed |
| Argument is ax+b | Multiply by a |
| Argument is a function of x | Multiply by its derivative |
| Two layers | Two factors, one per layer |
| Three layers | Three factors, one per layer |
| Inside is a constant | The derivative is zero, since the function is constant |
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
The chain rule is what keeps every other derivative rule alive inside a composite function. Differentiate the outside, leave the inside exactly as it stood, then multiply by the derivative of the inside. For f(g(x)) the result is f′(g(x))⋅g′(x), and that second factor is a multiplication, never an addition. The rule also stacks, because a function nested three deep hands you three factors in a chain. Almost every lost point on this topic is a second factor that never got written down, not a rule that was known incorrectly. The fix is procedural rather than conceptual: name the inside before you differentiate anything, then write the second factor before simplifying the first.
The chain rule is rarely a question by itself. It is the step that makes somebody else's question possible, and it sits inside the first line of tangent-line, rate-of-change, and implicit work. A missing factor there does not stay local; it changes every number that follows it.
Figure 1 — The chain rule produces two factors, not one. The outside function is differentiated at the untouched inside, and the derivative of the inside arrives as a multiplicand.
The only decision the rule requires is which layer is outside. Everything after that is bookkeeping.
| Function | Outside f | Inside g | Derivative |
|---|---|---|---|
| sin3x | sinu | 3x | 3cos3x |
| ex2 | eu | x2 | 2xex2 |
| (2x+1)5 | u5 | 2x+1 | 10(2x+1)4 |
| ln(x2+1) | lnu | x2+1 | x2+12x |
| 4x−3 | u1/2 | 4x−3 | 4x−32 |
The outside is the function whose argument is the whole inside expression. In sin2x the outside is the squaring and the inside is sinx; in 2sinx the outside is the exponential and the inside is sinx. Reading the expression aloud with the phrase "of" inserted at the right place settles the question in one pass.
Borrowed idea: The chain rule is one sentence, and it applies to every nested function you will meet. Differentiate the outside first, and leave the inside expression exactly as it was. Then multiply that result by the derivative of the inside, which is the second factor. Those two factors never merge into one, and neither of them may be dropped. Nearly every lost point in this topic is a second factor that never got written down. The rule stacks cleanly, because every additional layer contributes one more factor to the product.
| Exam disguise | What you see on the page | What your brain must translate | Required execution |
|---|---|---|---|
| A linear inside | sin5x, e3x, 2x+1 | The argument carries a constant slope | Multiply by that constant, even when it is 1 |
| A power inside | sin(x2), ex2 | The inside is not linear | Multiply by 2x, not by 2 |
| A function inside | sin(cosx), esinx | Two layers, so two factors | Differentiate outward, multiplying at each layer |
| A power of a function | sin2x, (x2+1)3 | The exponent is outside, the function is inside | Bring the power down, keep the inside, then multiply |
① Write f(x)=sin(3x) and differentiate it three times in a row, checking that a factor of 3 appears each time. The rule is the same motion repeated, and the repetition is what makes it stick.
② Take five expressions from your own homework that contain nested parentheses, and for each one write only the inside function, without differentiating. Naming the inside is the whole decision, and it takes about thirty seconds per expression.
③ On any tangent-line question, differentiate completely first and substitute second. Substituting into a half-differentiated expression silently discards the second factor, and the number you get afterwards looks entirely plausible.
Core Archetype (Simplest Form): Find dxd[sin(3x)+ex2].
Standard Execution:
- Split. The sum rule lets each term be handled alone.
- First term. The outside is sine, the inside is 3x, so the derivative is 3cos3x.
- Second term. The outside is the exponential, the inside is x2, so the derivative is 2xex2.
- Assemble. The derivative is 3cos3x+2xex2.
- Stop. No simplification is possible, and none is required.
Exam Variation (Context / Parameter Shift): Let f(x)=(2x+1)5. Find the equation of the line tangent to the graph of f at x=1.
Solution: The chain rule gives f′(x)=5(2x+1)4⋅2=10(2x+1)4. At x=1 the point is (1,243) and the slope is f′(1)=10⋅81=810. The tangent line is y−243=810(x−1). The factor of 2 inside the power is the entire difference between 810 and the 405 that a missing chain rule would produce.
Common Traps & Failure Signals:
6D Cognitive Error Diagnosis (Where Your Thinking Broke):
MCQ 1
If f(x)=sin(3x), what is f′(x)?
MCQ 2
Let g(x)=(2x+1)5. What is g′(0)?
MCQ 3
What is the derivative of h(x)=ex2?
FRQ 4
Let f be the function defined by f(x)=3x+1 for x≥−31.
(a) Find f′(x).
(b) Find the equation of the line tangent to the graph of f at x=1.
(c) The function g is defined by g(x)=f(x2). Find g′(x) and evaluate g′(1).
(d) A classmate writes f′(x)=23x+11 and claims the answer is complete. Identify the missing piece and explain what it changes about the slope at x=1.
| Move | |
|---|---|
| Argument is a bare x | No chain rule needed |
| Argument is ax+b | Multiply by a |
| Argument is a function of x | Multiply by its derivative |
| Two layers | Two factors, one per layer |
| Three layers | Three factors, one per layer |
| Inside is a constant | The derivative is zero, since the function is constant |
Strengthen this topic with AP-style practice, or continue to next topic.