Math lesson plan · 9th grade (Year 10 in England) · 45 minutes
Use aₙ = a₁ + (n − 1)d for arithmetic sequences
Algebra
CCSS HSF.BF.A.2CCSS HSF.LE.A.2England: GCSE Foundation Algebra
Objective and success criteria
Students will: Use aₙ = a₁ + (n − 1)d for arithmetic sequences.
- 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: Linear sequences: the nth term
Leads to: Decide if a sequence is arithmetic, geometric or neither; Sum an arithmetic series and find its length; Generate and model with recursive sequences
Materials: Balance scales or a balance drawing, Algebra tiles, Function machine cards, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Linear sequences: the nth term). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Find the nth term of the sequence 12, 17, 22, 27, … → 5n+7Prompt: The common difference tells you the number in front of n.
- The nth term of a sequence is 5n − 8. What is the 25th term? → 117Prompt: Substitute n = 25.
- Is 117 a term in the sequence with nth term 6n + 9? (Yes / No) → YesPrompt: Solve 6n + 9 = 117.
Hook · 5 min
- Show a "think of a number" puzzle and ask the class to work out the starting number.
- Share the objective: Use aₙ = a₁ + (n − 1)d for arithmetic sequences. 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: An arithmetic sequence starts 11, 17, 23, … Find the 75th term.
- Step 1: d = 6
- Step 2: a₇₅ = 11 + 74 × 6 = 455
- Answer: 455
- 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.
- An arithmetic sequence starts 24, 21, 18, … Find the 37th term. → -84Prompt: Find the common difference d.
- In an arithmetic sequence, a₃ = −11 and a₆ = −20. Find the common difference d. → -3Prompt: From term 3 to term 6 there are 3 steps of d.
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.
- The sequence 13, 9, 5, … continues with the same difference. Which term is equal to −191? → 52Prompt: Write the nth term formula.
- In an arithmetic sequence, a₇ = 9 and a₉ = 7. Find the first term a₁. → 15Prompt: From term 7 to term 9 there are 2 steps of d.
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: Linear sequences: the nth term.
- 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: Decide if a sequence is arithmetic, geometric or neither; Sum an arithmetic series and find its length; Generate and model with recursive sequences.
- 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-9.
Free lesson plan from eurekami.com/lesson-plan-builder