Math lesson plan · 11th grade (Year 12 in England) · 60 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 · 6 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 − 2| = 6. Give both solutions, separated by a comma. → x = -4 or x = 8Prompt: The expression inside can be 6 or −6.
- Solve: |3x + 21| = 15. Give both solutions, separated by a comma. → x = -12 or x = -2Prompt: Split into 3x + 21 = 15 and 3x + 21 = −15.
- Solve: 3|x − 7| + 4 = 13. Give both solutions, separated by a comma. → x = 4 or x = 10Prompt: Isolate the absolute value first, as if it were a single unknown.
Hook · 6 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 · 12 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 · 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.
- Solve: |x − 6| ≥ 7 (−1 < x < 13 / x ≤ −1 or x ≥ 13 / x ≤ −13 or x ≥ 1 / x ≥ 13) → x ≤ −1 or x ≥ 13Prompt: |x − a| is the distance from x to a.
- Solve: |4x − 12| ≤ 12 (x ≤ 0 or x ≥ 6 / 0 ≤ x ≤ 24 / −6 ≤ x ≤ 0 / 0 ≤ x ≤ 6) → 0 ≤ x ≤ 6Prompt: Rewrite as −12 ≤ 4x − 12 ≤ 12.
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.
- Solve: |2x + 2| ≤ 8 (−5 ≤ x ≤ 3 / −10 ≤ x ≤ 6 / −3 ≤ x ≤ 5 / x ≤ −5 or x ≥ 3) → −5 ≤ x ≤ 3Prompt: Rewrite as −8 ≤ 2x + 2 ≤ 8.
- Solve: |x| < 6 (−6 < x < 6 / x < 6 / x ≤ −6 or x ≥ 6) → −6 < x < 6Prompt: |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