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Derivative rules and domains

Read the idea, work independently, then explain what changed.

高二選擇性必修 第二册(A版).pdf · 5.2 · PDF 77 / printed page 72

Revisit first: Rates and tangent meaning

TOPIC 01

Derivative rules and domains

Choose power, product, quotient or chain rules from the expression structure, retaining domains.

What you will be able to explain

  • Choose power, product, quotient or chain rules from the expression structure, retaining domains.
  • Recover an unknown inner rate and audit missing derivative factors.

Differentiate the structure

Multiplying or nesting quantities requires tracing where their changes come from.

A product changes through either factor; a composition passes the rate through its inner input.

(uv)′=u′v+uv′; (f∘g)′=f′(g)g′(uv)'=u'v+uv';\ (f\circ g)'=f'(g)g'
  1. A product changes through either factor; a composition passes the rate through its inner input.

    Δ(uv)=uΔv+vΔu+ΔuΔv\Delta(uv)=u\Delta v+v\Delta u+\Delta u\Delta v
  2. A product changes through either factor; a composition passes the rate through its inner input.

    (u/v)′=(u′v−uv′)/v2(u/v)'=(u'v-uv')/v^2

When it applies

  • Both component derivatives must exist; quotient denominators stay nonzero.
  • Real logarithms require positive arguments; trigonometric derivatives here use radians.

Counterexample: For x·x, multiplying derivatives gives 1, but the true derivative is 2x.

  • Expression tree, expanded polynomial, inner-rate table and derivative curve.

PREDICT → EXPLORE → EXPLAIN → TRANSFER

Make a prediction, then explore the relationship.

Lesson question: For (kx+1)³, predict the derivative at zero when k changes sign.

Model exploration: predict which displayed result changes with the parameters, check the values, and compare the observation with the lesson question.

The chain rule multiplies by k=2; the derivative at x=0 is 6.-2-35.85-112.825061.51110.1752158.85xf 藍/blue; f′ 綠/green; missing k 漏 k 橙/orange

The chain rule multiplies by k=2; the derivative at x=0 is 6.

Numerical data for the diagram; printable
InputFunctionDerivativeMissing factor result
-1-163
0163
1275427

Explain: Compare the correct 3k(kx+1)² with the expression missing k.

Transfer: At k=0, explain why the function is constant and its derivative vanishes.

Try it. Leave your reasoning visible.

Use one hint at a time. A correction explains what changed, not just the final answer.

Examples 1–3: study a full demonstration; finish a solution from its first step; then choose a method independently before opening the solution. The remaining exercises may continue after class.

01 / Foundation#Worked example

Find f′(2) for f=3x⁴−2x+5.

f(x)=3x4−2x+5f(x)=3x^4-2x+5
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Differentiate term by term; a constant differentiates to zero.
Hint 2
Start from this intermediate relation.
f′=12x3−2f'=12x^3-2
Worked solution
  1. Differentiate term by term; a constant differentiates to zero.

  2. Establish and simplify this mathematical relation.

    f′=12x3−2f'=12x^3-2
  3. Establish and simplify this mathematical relation.

    f′(2)=96−2=94f'(2)=96-2=94
  4. Keep the exponent decrease as well as its multiplier.

The requested value is 94.

Checks and common pitfalls: Keep the exponent decrease as well as its multiplier.

Think first. Reveal a hint when the class is ready.

02 / Standard#Worked example

Find the derivative of (x²+1)(x−2).

f=(x2+1)(x−2)f=(x^2+1)(x-2)

First step provided — complete the reasoning:Each factor contributes a derivative term.

  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Each factor contributes a derivative term.
Hint 2
Start from this intermediate relation.
f′=2x(x−2)+(x2+1)f'=2x(x-2)+(x^2+1)
Worked solution
  1. Each factor contributes a derivative term.

  2. Establish and simplify this mathematical relation.

    f′=2x(x−2)+(x2+1)f'=2x(x-2)+(x^2+1)
  3. Establish and simplify this mathematical relation.

    f′=3x2−4x+1f'=3x^2-4x+1
  4. The product of the derivatives would omit both cross contributions.

f′=3x²−4x+1.

Checks and common pitfalls: The product of the derivatives would omit both cross contributions.

Reasoning checklist · self / teacher assessment
  • Each factor contributes a derivative term.
  • Give the intermediate relations and explain the conclusion with its restrictions.
  • The product of the derivatives would omit both cross contributions.

Think first. Reveal a hint when the class is ready.

03 / Transfer#Worked example

For F(x)=g(x²), F′(2)=24. Find g′(4) and explain why g′(4)=24 is wrong.

F′(2)=24; F(x)=g(x2)F'(2)=24;\ F(x)=g(x^2)
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Undo the inner derivative factor.
Hint 2
Start from this intermediate relation.
F′(x)=2xg′(x2)F'(x)=2xg'(x^2)
Worked solution
  1. Undo the inner derivative factor.

  2. Establish and simplify this mathematical relation.

    F′(x)=2xg′(x2)F'(x)=2xg'(x^2)
  3. Establish and simplify this mathematical relation.

    24=4g′(4)  ⟹  g′(4)=624=4g'(4)\implies g'(4)=6
  4. F′ and g′ are derivatives with respect to different inputs.

The requested value is 6.

Checks and common pitfalls: F′ and g′ are derivatives with respect to different inputs.

Think first. Reveal a hint when the class is ready.

04 / Foundation#Your turn

Find the derivative of the constant function f(x)=7.

f(x)=7f(x)=7
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
No output change occurs for any nonzero input increment.
Hint 2
Start from this intermediate relation.
[f(x+h)−f(x)]/h=(7−7)/h[f(x+h)-f(x)]/h=(7-7)/h
Worked solution
  1. No output change occurs for any nonzero input increment.

  2. Establish and simplify this mathematical relation.

    [f(x+h)−f(x)]/h=(7−7)/h[f(x+h)-f(x)]/h=(7-7)/h
  3. Establish and simplify this mathematical relation.

    f′(x)=0f'(x)=0
  4. A constant derivative is zero, not the same constant.

The requested value is 0.

Checks and common pitfalls: A constant derivative is zero, not the same constant.

Think first. Reveal a hint when the class is ready.

05 / Foundation#Your turn

Find f′(−1) for f=x⁵.

f=x5f=x^5
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Apply the power rule and evaluate the even power.
Hint 2
Start from this intermediate relation.
f′=5x4f'=5x^4
Worked solution
  1. Apply the power rule and evaluate the even power.

  2. Establish and simplify this mathematical relation.

    f′=5x4f'=5x^4
  3. Establish and simplify this mathematical relation.

    f′(−1)=5f'(-1)=5
  4. A negative input does not force a negative derivative.

The requested value is 5.

Checks and common pitfalls: A negative input does not force a negative derivative.

Think first. Reveal a hint when the class is ready.

06 / Foundation#Your turn

Find f′(0) for f=(2x+1)³.

f=(2x+1)3f=(2x+1)^3
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Differentiate the outside and multiply by the inside derivative.
Hint 2
Start from this intermediate relation.
f′=3(2x+1)2(2)f'=3(2x+1)^2(2)
Worked solution
  1. Differentiate the outside and multiply by the inside derivative.

  2. Establish and simplify this mathematical relation.

    f′=3(2x+1)2(2)f'=3(2x+1)^2(2)
  3. Establish and simplify this mathematical relation.

    f′(0)=6f'(0)=6
  4. Omitting the inner factor gives half the correct rate.

The requested value is 6.

Checks and common pitfalls: Omitting the inner factor gives half the correct rate.

Think first. Reveal a hint when the class is ready.

07 / Foundation#Your turn

Find f′(2) for f=(x+1)/(x−1).

f=(x+1)/(x−1); x≠1f=(x+1)/(x-1);\ x\ne1
  • The rational denominator must be nonzero; excluded inputs are not points on its curve.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Subtract the numerator terms in the quotient-rule order.
Hint 2
Start from this intermediate relation.
f′=[(x−1)−(x+1)]/(x−1)2f'=[(x-1)-(x+1)]/(x-1)^2
Worked solution
  1. Subtract the numerator terms in the quotient-rule order.

  2. Establish and simplify this mathematical relation.

    f′=[(x−1)−(x+1)]/(x−1)2f'=[(x-1)-(x+1)]/(x-1)^2
  3. Establish and simplify this mathematical relation.

    f′(2)=−2f'(2)=-2
  4. Dividing the derivatives would give 1 and is not a quotient rule.

The requested value is -2.

Checks and common pitfalls: Dividing the derivatives would give 1 and is not a quotient rule.

Think first. Reveal a hint when the class is ready.

08 / Standard#Your turn

Differentiate ln(3x−2), stating its real domain.

f=ln⁡(3x−2)f=\ln(3x-2)
  • Use real x>2/3 so 3x−2>0.
Skills and prerequisite lessons

Scope: School extension · This task develops the school textbook skill beyond the explicitly listed JAE syllabus topics.

Working and explanation

BUILD THE REASONING

Hint 1
Retain the positive argument condition alongside the chain rule.
Hint 2
Start from this intermediate relation.
f′=3/(3x−2)f'=3/(3x-2)
Worked solution
  1. Retain the positive argument condition alongside the chain rule.

  2. Establish and simplify this mathematical relation.

    f′=3/(3x−2)f'=3/(3x-2)
  3. Establish and simplify this mathematical relation.

    3x−2>0  ⟹  x>2/33x-2>0\implies x>2/3
  4. A nonzero denominator alone is weaker than the logarithm domain.

f′=3/(3x−2), x>2/3.

Checks and common pitfalls: A nonzero denominator alone is weaker than the logarithm domain.

Reasoning checklist · self / teacher assessment
  • Retain the positive argument condition alongside the chain rule.
  • Give the intermediate relations and explain the conclusion with its restrictions.
  • A nonzero denominator alone is weaker than the logarithm domain.

Think first. Reveal a hint when the class is ready.

09 / Standard#Your turn

Find the derivative of sin(2x), where x is in radians.

f=sin⁡(2x)f=\sin(2x)
  • The trigonometric variable is measured in radians.
Skills and prerequisite lessons

Scope: School extension · This task develops the school textbook skill beyond the explicitly listed JAE syllabus topics.

Working and explanation

BUILD THE REASONING

Hint 1
Use the radians derivative and the inner factor 2.
Hint 2
Start from this intermediate relation.
f′=cos⁡(2x)(2)f'=\cos(2x)(2)
Worked solution
  1. Use the radians derivative and the inner factor 2.

  2. Establish and simplify this mathematical relation.

    f′=cos⁡(2x)(2)f'=\cos(2x)(2)
  3. Establish and simplify this mathematical relation.

    f′=2cos⁡(2x)f'=2\cos(2x)
  4. Degree-based variables introduce an additional π/180 factor.

f′=2cos(2x).

Checks and common pitfalls: Degree-based variables introduce an additional π/180 factor.

Reasoning checklist · self / teacher assessment
  • Use the radians derivative and the inner factor 2.
  • Give the intermediate relations and explain the conclusion with its restrictions.
  • Degree-based variables introduce an additional π/180 factor.

Think first. Reveal a hint when the class is ready.

10 / Standard#Your turn

Find the derivative of e^(−x).

f=e−xf=e^{-x}
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: School extension · This task develops the school textbook skill beyond the explicitly listed JAE syllabus topics.

Working and explanation

BUILD THE REASONING

Hint 1
The inside function −x has derivative −1.
Hint 2
Start from this intermediate relation.
f′=e−x(−1)f'=e^{-x}(-1)
Worked solution
  1. The inside function −x has derivative −1.

  2. Establish and simplify this mathematical relation.

    f′=e−x(−1)f'=e^{-x}(-1)
  3. Establish and simplify this mathematical relation.

    f′=−e−xf'=-e^{-x}
  4. The exponential remains positive although its rate here is negative.

f′=−e^(−x).

Checks and common pitfalls: The exponential remains positive although its rate here is negative.

Reasoning checklist · self / teacher assessment
  • The inside function −x has derivative −1.
  • Give the intermediate relations and explain the conclusion with its restrictions.
  • The exponential remains positive although its rate here is negative.

Think first. Reveal a hint when the class is ready.

11 / Standard#Your turn

Find f′(1) for f=x²(x−3).

f=x2(x−3)f=x^2(x-3)
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Differentiate both factors or expand first.
Hint 2
Start from this intermediate relation.
f′=2x(x−3)+x2f'=2x(x-3)+x^2
Worked solution
  1. Differentiate both factors or expand first.

  2. Establish and simplify this mathematical relation.

    f′=2x(x−3)+x2f'=2x(x-3)+x^2
  3. Establish and simplify this mathematical relation.

    f′(1)=−4+1=−3f'(1)=-4+1=-3
  4. Substitute only after the full derivative has been formed.

The requested value is -3.

Checks and common pitfalls: Substitute only after the full derivative has been formed.

Think first. Reveal a hint when the class is ready.

12 / Transfer#Your turn

A student differentiates x·x as 1·1. Find the missing terms and explain the error.

f=x⋅xf=x\cdot x
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Track a change in either factor while the other remains.
Hint 2
Start from this intermediate relation.
Δ(x2)=2xΔx+(Δx)2\Delta(x^2)=2x\Delta x+(\Delta x)^2
Worked solution
  1. Track a change in either factor while the other remains.

  2. Establish and simplify this mathematical relation.

    Δ(x2)=2xΔx+(Δx)2\Delta(x^2)=2x\Delta x+(\Delta x)^2
  3. Establish and simplify this mathematical relation.

    f′=lim⁡h→0(2x+h)=2xf'=\lim_{h\to0}(2x+h)=2x
  4. For example x=0 gives derivative 0 rather than 1.

f′=1·x+x·1=2x; derivative products are not the product rule.

Checks and common pitfalls: For example x=0 gives derivative 0 rather than 1.

Reasoning checklist · self / teacher assessment
  • Track a change in either factor while the other remains.
  • Give the intermediate relations and explain the conclusion with its restrictions.
  • For example x=0 gives derivative 0 rather than 1.

Think first. Reveal a hint when the class is ready.

13 / Transfer#Your turn

For f(x)=(ax+1)³, find all real a such that f′(0)=12.

f=(ax+1)3f=(ax+1)^3
  • Use the stated real domain and distinguish stationary candidates, attained extrema and actual concavity changes.
Skills and prerequisite lessons

Scope: JM02 · This task uses elementary probability rules or polynomial calculus named in the JAE syllabus.

Working and explanation

BUILD THE REASONING

Hint 1
Express the requested derivative in terms of the unknown parameter.
Hint 2
Start from this intermediate relation.
f′(x)=3a(ax+1)2f'(x)=3a(ax+1)^2
Worked solution
  1. Express the requested derivative in terms of the unknown parameter.

  2. Establish and simplify this mathematical relation.

    f′(x)=3a(ax+1)2f'(x)=3a(ax+1)^2
  3. Establish and simplify this mathematical relation.

    f′(0)=3a=12  ⟹  a=4f'(0)=3a=12\implies a=4
  4. The inner derivative depends on a, not on x alone.

The requested value is 4.

Checks and common pitfalls: The inner derivative depends on a, not on x alone.

Think first. Reveal a hint when the class is ready.

Focus on one question

End-of-lesson check and correction

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    Choose a foundation skill to revisit ↗

    Teacher preparation and assessment

    40-minute lesson

    MinutesFocusTeaching and student action
    5PredictPredict the derivative of x·x before expanding.
    8Build the ideaSample response: each factor contributes 1·x, so the sum is 2x.
    10Guided comparisonSample response: F′(2)=4g′(4)=24, so g′(4)=6.
    12Independent practice and discussionSort tasks by their outermost operation, then contrast polynomial and extension domains.
    5Exit and next stepExit: (3x+1)² differentiates to 6(3x+1), and ln(3x−2) needs x>2/3.

    Question sequence

    • Predict the derivative of x·x before expanding.
    • Sample response: F′(2)=4g′(4)=24, so g′(4)=6.

    Board development

    • Differentiate the structure
      (uv)′=u′v+uv′; (f∘g)′=f′(g)g′(uv)'=u'v+uv';\ (f\circ g)'=f'(g)g'

    Expected correct and incorrect responses

    • Correct sample: 6
    • Anticipated wrong response: 2 — Omits the inner derivative in a composition.
    • Anticipated wrong response: 6x³ — Multiplies derivatives instead of adding the two product-rule contributions.
    • Omitting the inner derivative.
    • Multiplying or dividing derivatives as if the original arithmetic rules still apply.

    Follow-up questions

    • Follow-up: Keep both the outer derivative and inner derivative. Then explain why 2 fails.
    • Follow-up: First combine the powers, then differentiate. Then explain why 6x³ fails.
    • At k=0, explain why the function is constant and its derivative vanishes.

    Remediation

    • Sample response: each factor contributes 1·x, so the sum is 2x.
    • Return to the first example, identify the missing condition, then solve one fresh task without viewing its solution.

    Exit check

    • Exit: (3x+1)² differentiates to 6(3x+1), and ln(3x−2) needs x>2/3.

    Assessment checklist and sample responses

    • Representation and conditions
      Both factors are differentiable; (uv)′=u′v+uv′.
    • Connected mathematical reasoning
      For (x²+1)(x−2), derivative 2x(x−2)+(x²+1)=3x²−4x+1.
    • Interpretation and boundary check
      ln(3x−2) needs x>2/3, not merely x≠2/3.

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    Curriculum and source notes ↗