Math lesson plan · 11th grade (Year 12 in England) · 30 minutes
Evaluate logarithms without a calculator
Algebra
CCSS HSF.BF.B.5CCSS HSF.LE.A.4England: A-level Pure Mathematics
Objective and success criteria
Students will: Evaluate logarithms without a calculator.
- 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: Logarithms
Leads to: Combine logarithms into a single logarithm; Evaluate logarithms with the change-of-base formula
Materials: Balance scales or a balance drawing, Algebra tiles, Function machine cards, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 4 min
- Quick retrieval from earlier learning (Logarithms). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Evaluate log_5(1/25). → -2Prompt: Ask: 5 to what power gives 1/25?
- Kwame is revising the product law of logarithms. Which statement is true? (log(ab) = log a + log b / log(a/b) = log a ÷ log b / log(ab) = log a × log b / log(a + b) = log a + log b) → log(ab) = log a + log bPrompt: Logs turn multiplication into addition.
- Evaluate log_5(625) − log_5(5). → 3Prompt: Use log a − log b = log(a/b).
Hook · 3 min
- Show a "think of a number" puzzle and ask the class to work out the starting number.
- Share the objective: Evaluate logarithms without a calculator. Read the success criteria together.
I do: model it · 7 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: Evaluate log₂ 32.
- Step 1: 32 = 2^5
- Step 2: log₂ 32 = 5
- Answer: 5
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 6 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- Evaluate log₂ 16. → 4Prompt: log₂ N asks: 2 to what power gives N?
- Evaluate log₂ (1/2). → -1Prompt: A fraction 1/bⁿ is b to a negative power.
You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.
Exit ticket · 3 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Evaluate log₄ 16. → 2Prompt: log₄ N asks: 4 to what power gives N?
- Evaluate log₂ (1/4). → -2Prompt: A fraction 1/bⁿ is b to a negative power.
Watch out for
- Doing an operation to one side of an equation but not the other.
- Reading 3x as 3 + x, or thinking x always stands for one particular number.
Support and stretch
Support
- Review first: Logarithms.
- Use concrete materials: balance scales or a balance drawing.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Combine logarithms into a single logarithm; Evaluate logarithms with the change-of-base formula.
- 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-11.
Free lesson plan from eurekami.com/lesson-plan-builder