Math lesson plan · 7th grade (Year 8 in England) · 45 minutes
Find complementary and supplementary angles
Geometry
CCSS 7.G.B.5England: KS3 Geometry and measures
Objective and success criteria
Students will: Find complementary and supplementary angles.
- 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: Vertical, complementary and supplementary angles
Leads to: Angles on a straight line and around a point
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Vertical, complementary and supplementary angles). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- What is the supplement of 54°? → 126Prompt: Supplementary angles add up to 180°.
- Two angles on a straight line are (2x + 32)° and 108°. Find x. → x = 20Prompt: Angles on a straight line add up to 180°.
- Two straight lines cross. One angle is 165°. What is the vertically opposite angle? → 165Prompt: Vertically opposite angles are equal.
Hook · 5 min
- Show a photo of a building, a tile pattern or a road sign and ask what shapes and angles students can spot.
- Share the objective: Find complementary and supplementary angles. 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: What is the complement of a 14° angle?
- Step 1: 90° − 14° = 76°
- Answer: 76
- 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.
- What is the complement of a 65° angle? → 25Prompt: Complementary angles add to 90°; supplementary angles add to 180°.
- Two angles are complementary. One is x° and the other is (3x + 38)°. Find x. → 13Prompt: The two angles add to 90°.
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.
- Omar says the complement of 50° is 130°. What mistake did Omar make, and what is the correct complement? → 40Prompt: Omar used 180°, which is for supplements.
- Two angles are supplementary. One is x° and the other is (3x + 120)°. Find x. → 15Prompt: The two angles add to 180°.
Watch out for
- Thinking a square turned on its corner is no longer a square.
- Reading the wrong scale on a protractor.
Support and stretch
Support
- Review first: Vertical, complementary and supplementary angles.
- Use concrete materials: rulers and protractors.
- 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: Angles on a straight line and around a point.
- 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-7.
Free lesson plan from eurekami.com/lesson-plan-builder