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Math lesson plan · 12th grade (Year 13 in England) · 45 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 · 5 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 = 3x³ + 2x² − 4x − 4 at x = 2. → 40Prompt: Differentiate first.
  2. Find the gradient of y = x² + 4/x at x = 1. → -2Prompt: 4/x = 4x⁻¹, which differentiates to −4x⁻².
  3. At what x is the gradient of y = x² − 2x + 6 equal to −6? → x = -2Prompt: Differentiate, then set dy/dx equal to the gradient.

Hook · 5 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 · 10 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 · 10 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′(8) = −2. What does this mean? (The average rate over the whole period / The battery has 2% left / At that moment the battery is falling by about 2 percentage points per hour / The total change since the start) → At that moment the battery is falling by about 2 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) = 2x² + 51x + 200. Use C′(9) to estimate the extra cost of making one more case after 9. → 87Prompt: The derivative gives the cost per extra case at that point (marginal cost).

You do: independent practice · 10 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 · 5 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) = 3x² + 27x + 200. Use C′(38) to estimate the extra cost of making one more case after 38. → 255Prompt: The derivative gives the cost per extra case at that point (marginal cost).
  2. V(t) is the number of users of an app after t days. V′(20) = 25. What does this mean? (There are 25 users on day 20 / On day 20, users are increasing at about 25 per day / The total change since the start / The average rate over the whole period) → On day 20, users are increasing at about 25 per dayPrompt: 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

    B(h) is a phone's battery level (%) after h hours. B′(8) = −8. What does this mean?

    • A. At that moment the battery is falling by about 8 percentage points per hour
    • B. The battery has 8% left
    • C. The average rate over the whole period
    • D. The total change since the start

  2. Question 2

    The cost (in dollars) of making x phone cases is C(x) = x² + 52x + 200. Use C′(33) to estimate the extra cost of making one more case after 33.

    Answer

  3. Question 3

    T(m) is the temperature (°C) of a drink m minutes after it is made. T′(16) = −4. What does this mean?

    • A. At 16 minutes, the drink is cooling at about 4 °C per minute
    • B. The average rate over the whole period
    • C. The drink is at 4 °C after 16 minutes
    • D. The total change since the start

  4. Question 4

    The cost (in dollars) of making x phone cases is C(x) = 4x² + 51x + 200. Use C′(11) to estimate the extra cost of making one more case after 11.

    Answer

  5. Question 5

    The cost (in dollars) of making x phone cases is C(x) = 2x² + 40x + 200. Use C′(24) to estimate the extra cost of making one more case after 24.

    Answer

  6. Question 6

    B(h) is a phone's battery level (%) after h hours. B′(4) = −8. What does this mean?

    • A. The average rate over the whole period
    • B. The total change since the start
    • C. At that moment the battery is falling by about 8 percentage points per hour
    • D. The battery has 8% left

  7. Question 7

    The cost (in dollars) of making x phone cases is C(x) = 4x² + 54x + 200. Use C′(9) to estimate the extra cost of making one more case after 9.

    Answer

  8. Question 8

    V(t) is the number of users of an app after t days. V′(8) = 17. What does this mean?

    • A. The total change since the start
    • B. The average rate over the whole period
    • C. On day 8, users are increasing at about 17 per day
    • D. There are 17 users on day 8

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

  1. 1. A. At that moment the battery is falling by about 8 percentage points per hour

  2. 2. 118

  3. 3. A. At 16 minutes, the drink is cooling at about 4 °C per minute

  4. 4. 139

  5. 5. 136

  6. 6. C. At that moment the battery is falling by about 8 percentage points per hour

  7. 7. 126

  8. 8. C. On day 8, users are increasing at about 17 per day