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Math lesson plan · 12th grade (Year 13 in England) · 60 minutes

Interpret a derivative as a rate of change in context

Calculus

CCSS HSF.IF.B.6CCSS HSF.IF.B.4England: A-level Pure Mathematics

Objective and success criteria

Students will: Interpret a derivative as a rate of change in context.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Find the gradient of a curve at a point

Leads to: the next skill in this area.

Materials: Graphing tool or graph paper, Scientific calculators, Mini whiteboards and pens, Printed student sheet (below).

Warm-up · 6 min

  • Quick retrieval from earlier learning (Find the gradient of a curve at a point). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Find the gradient of y = x³ − 2x² − 2x − 9 at x = −3. → 37Prompt: Differentiate first.
  2. Find the gradient of y = x² + 3/x at x = −1. → -5Prompt: 3/x = 3x⁻¹, which differentiates to −3x⁻².
  3. At what x is the gradient of y = x² + 6x − 1 equal to 16? → x = 5Prompt: Differentiate, then set dy/dx equal to the gradient.

Hook · 6 min

  • Show a distance-time graph of a journey and ask how fast it was going at one exact moment.
  • Share the objective: Interpret a derivative as a rate of change in context. Read the success criteria together.

I do: model it · 12 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: V(t) is the number of users of an app after t days. V′(15) = 32. What does this mean?
  • Step 1: On day 15, users are increasing at about 32 per day
  • Answer: On day 15, users are increasing at about 32 per day
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 12 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. B(h) is a phone's battery level (%) after h hours. B′(4) = −3. What does this mean? (The average rate over the whole period / The total change since the start / At that moment the battery is falling by about 3 percentage points per hour / The battery has 3% left) → At that moment the battery is falling by about 3 percentage points per hourPrompt: A derivative is a rate: units of output per unit of input.
  2. The cost (in dollars) of making x phone cases is C(x) = 3x² + 59x + 200. Use C′(12) to estimate the extra cost of making one more case after 12. → 131Prompt: The derivative gives the cost per extra case at that point (marginal cost).

You do: independent practice · 17 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. The cost (in dollars) of making x phone cases is C(x) = x² + 15x + 200. Use C′(20) to estimate the extra cost of making one more case after 20. → 55Prompt: The derivative gives the cost per extra case at that point (marginal cost).
  2. B(h) is a phone's battery level (%) after h hours. B′(5) = −5. What does this mean? (The average rate over the whole period / The battery has 5% left / At that moment the battery is falling by about 5 percentage points per hour / The total change since the start) → At that moment the battery is falling by about 5 percentage points per hourPrompt: A derivative is a rate: units of output per unit of input.

Watch out for

  • Forgetting the constant of integration.
  • Differentiating a product by multiplying the derivatives.

Support and stretch

Support

  • Review first: Find the gradient of a curve at a point.
  • Use concrete materials: graphing tool or graph paper.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-12.

Free lesson plan from eurekami.com/lesson-plan-builder

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Interpret a derivative as a rate of change in context

Name

Date

Score/ 8

  1. Question 1

    T(m) is the temperature (°C) of a drink m minutes after it is made. T′(18) = −1. What does this mean?

    • A. The total change since the start
    • B. The average rate over the whole period
    • C. At 18 minutes, the drink is cooling at about 1 °C per minute
    • D. The drink is at 1 °C after 18 minutes

  2. Question 2

    The cost (in dollars) of making x phone cases is C(x) = x² + 48x + 200. Use C′(20) to estimate the extra cost of making one more case after 20.

    Answer

  3. Question 3

    B(h) is a phone's battery level (%) after h hours. B′(4) = −3. What does this mean?

    • A. The battery has 3% left
    • B. The total change since the start
    • C. The average rate over the whole period
    • D. At that moment the battery is falling by about 3 percentage points per hour

  4. Question 4

    The cost (in dollars) of making x phone cases is C(x) = 2x² + 36x + 200. Use C′(33) to estimate the extra cost of making one more case after 33.

    Answer

  5. Question 5

    B(h) is a phone's battery level (%) after h hours. B′(7) = −5. What does this mean?

    • A. The average rate over the whole period
    • B. The battery has 5% left
    • C. At that moment the battery is falling by about 5 percentage points per hour
    • D. The total change since the start

  6. Question 6

    The cost (in dollars) of making x phone cases is C(x) = x² + 27x + 200. Use C′(12) to estimate the extra cost of making one more case after 12.

    Answer

  7. Question 7

    V(t) is the number of users of an app after t days. V′(24) = 37. What does this mean?

    • A. There are 37 users on day 24
    • B. The average rate over the whole period
    • C. The total change since the start
    • D. On day 24, users are increasing at about 37 per day

  8. Question 8

    The cost (in dollars) of making x phone cases is C(x) = 4x² + 28x + 200. Use C′(8) to estimate the extra cost of making one more case after 8.

    Answer

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Answer key: Interpret a derivative as a rate of change in context

  1. 1. C. At 18 minutes, the drink is cooling at about 1 °C per minute

  2. 2. 88

  3. 3. D. At that moment the battery is falling by about 3 percentage points per hour

  4. 4. 168

  5. 5. C. At that moment the battery is falling by about 5 percentage points per hour

  6. 6. 51

  7. 7. D. On day 24, users are increasing at about 37 per day

  8. 8. 92