Math lesson plan · 11th grade (Year 12 in England) · 30 minutes
Solve absolute value inequalities
Algebra
CCSS HSA.REI.B.3
Objective and success criteria
Students will: Solve absolute value inequalities.
- 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: Solve absolute value equations |ax + b| = c; Solve 'or' compound inequalities
Leads to: the next skill in this area.
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 (Solve absolute value equations |ax + b| = c; Solve 'or' compound inequalities). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Solve: |x − 9| = 8. Give both solutions, separated by a comma. → x = 1 or x = 17Prompt: The expression inside can be 8 or −8.
- Solve: |2x − 6| = 2. Give both solutions, separated by a comma. → x = 2 or x = 4Prompt: Split into 2x − 6 = 2 and 2x − 6 = −2.
- Solve: 3|x + 6| − 9 = 15. Give both solutions, separated by a comma. → x = -14 or x = 2Prompt: Isolate the absolute value first, as if it were a single unknown.
Hook · 3 min
- Show a "think of a number" puzzle and ask the class to work out the starting number.
- Share the objective: Solve absolute value inequalities. 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: Solve: |x + 5| ≥ 3
- Step 1: x ≤ −8 or x ≥ −2
- Answer: x ≤ −8 or x ≥ −2
- 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.
- Solve: |x + 5| < 8 (−13 < x < 3 / x ≤ −13 or x ≥ 3 / −3 < x < 13 / x < 3) → −13 < x < 3Prompt: |x − a| is the distance from x to a.
- Solve: |3x + 3| ≤ 9 (−2 ≤ x ≤ 4 / −4 ≤ x ≤ 2 / x ≤ −4 or x ≥ 2 / −12 ≤ x ≤ 6) → −4 ≤ x ≤ 2Prompt: Rewrite as −9 ≤ 3x + 3 ≤ 9.
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.
- Solve: |3x| ≤ 9 (x ≤ −3 or x ≥ 3 / −3 ≤ x ≤ 3 / −9 ≤ x ≤ 9) → −3 ≤ x ≤ 3Prompt: Rewrite as −9 ≤ 3x ≤ 9.
- Solve: |x + 6| < 7 (−13 < x < 1 / x ≤ −13 or x ≥ 1 / x < 1 / −1 < x < 13) → −13 < x < 1Prompt: |x − a| is the distance from x to a.
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: Solve absolute value equations |ax + b| = c.
- 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
- 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