Math lesson plan · 8th grade (Year 9 in England) · 60 minutes
Interior angles of polygons
Geometry
CCSS 8.G.A.5England: KS3 Geometry and measuresEngland: GCSE Foundation Geometry and measures
Objective and success criteria
Students will: Interior angles of polygons.
- 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: Interior and exterior angles of polygons
Leads to: Exterior angles of regular polygons
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 6 min
- Quick retrieval from earlier learning (Interior and exterior angles of polygons). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- What is the sum of the interior angles of a polygon with 9 sides, in degrees? → 1260Prompt: Split the polygon into triangles from one vertex.
- Each exterior angle of a regular polygon is 12°. How many sides does it have? → 30Prompt: All the exterior angles add up to 360°.
- What is the sum of the interior angles of a polygon with 10 sides, in degrees? → 1440Prompt: Split the polygon into triangles from one vertex.
Hook · 6 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: Interior angles of polygons. 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: A 5-sided polygon has angles 105°, 102°, 112°, 106° and x°. Find x.
- Step 1: (5 − 2) × 180 = 540
- Step 2: 540 − 425 = 115°
- Answer: 115
- 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.
- A 5-sided polygon has angles 106°, 109°, 113°, 107° and x°. Find x. → 105Prompt: The interior angles add to (5 − 2) × 180°.
- A 5-sided polygon has angles 102°, 113°, 105°, 113° and x°. Find x. → 107Prompt: The interior angles add to (5 − 2) × 180°.
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.
- A 6-sided polygon has angles 119°, 117°, 114°, 123°, 116° and x°. Find x. → 131Prompt: The interior angles add to (6 − 2) × 180°.
- A 8-sided polygon has angles 140°, 136°, 131°, 141°, 135°, 132°, 134° and x°. Find x. → 131Prompt: The interior angles add to (8 − 2) × 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: Interior and exterior angles of polygons.
- 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: Exterior angles of regular polygons.
- 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-8.
Free lesson plan from eurekami.com/lesson-plan-builder