Math lesson plan · 10th grade (Year 11 in England) · 30 minutes
Find angles in isosceles triangles
Geometry
CCSS HSG.CO.C.10England: GCSE Foundation Geometry
Objective and success criteria
Students will: Find angles in isosceles triangles.
- 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: Classify triangles; angles in a triangle
Leads to: the next skill in this area.
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 4 min
- Quick retrieval from earlier learning (Classify triangles; angles in a triangle). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- A triangle has exactly two sides equal. What type of triangle is it? (right-angled / scalene / equilateral / isosceles) → isoscelesPrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
- An isosceles triangle has two equal base angles of 31° each. What is the third angle, in degrees? → 118Prompt: The three angles add to 180°.
- A triangle has no equal sides. What type of triangle is it? (isosceles / scalene / equilateral / right-angled) → scalenePrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
Hook · 3 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 angles in isosceles triangles. 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: An isosceles triangle has an angle of 64° between its two equal sides. Find one of the other angles.
- Step 1: (180° − 64°) ÷ 2 = 58°
- Answer: 58
- 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.
- An isosceles triangle has an angle of 98° between its two equal sides. Find one of the other angles. → 41Prompt: The two base angles are equal.
- In isosceles triangle ABC, AB = AC and ∠B = 65°. Find ∠A. → 50Prompt: AB = AC means the angles opposite them, ∠C and ∠B, are equal.
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.
- In isosceles triangle PQR, PQ = PR. Side QR is extended beyond R to S. ∠PRS = 129°. Find ∠QPR. → 78Prompt: ∠PRQ and ∠PRS lie on a straight line.
- In isosceles triangle ABC, AB = AC and ∠B = 31°. Find ∠A. → 118Prompt: AB = AC means the angles opposite them, ∠C and ∠B, are equal.
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: Classify triangles; angles in a triangle.
- 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
- 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-10.
Free lesson plan from eurekami.com/lesson-plan-builder