Back to the lesson plan builder
Lesson length30 min45 min60 min
Questions🔀 New questions

Print the plan for yourself; the student sheet prints on its own page after it.

Math lesson plan · 7th grade (Year 8 in England) · 30 minutes

Missing angles in triangles, including isosceles and algebra

Geometry

CCSS 7.G.A.2CCSS 8.G.A.5England: KS3 Geometry and measuresEngland: GCSE Foundation Geometry and measures

Objective and success criteria

Students will: Missing angles in triangles, including isosceles and algebra.

  • 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; Two-step equations

Leads to: Use the exterior angle of a triangle

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; Two-step equations). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. A triangle has all three sides equal. What type of triangle is it? (equilateral / isosceles / scalene / right-angled) → equilateralPrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
  2. A triangle has no equal sides. What type of triangle is it? (scalene / right-angled / equilateral / isosceles) → scalenePrompt: Equilateral: all sides equal. Isosceles: two equal. Scalene: none equal.
  3. A triangle has one angle of 90°. What type of triangle is it? (equilateral / scalene / isosceles / right-angled) → right-angledPrompt: 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: Missing angles in triangles, including isosceles and algebra. 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: The angles of a triangle are x, 3x and 5x. What is the largest angle, in degrees?
  • Step 1: 9x = 180
  • Step 2: x = 20
  • Step 3: 5 × 20 = 100°
  • Answer: 100
  • 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.
  1. The angles of a triangle are 2x, 2x and 5x. What is the largest angle, in degrees? → 100Prompt: They add to 180°: 9x = 180.
  2. In a triangle, two angles are 68° and 25°. One side is extended to make an exterior angle next to the third angle. How big is that exterior angle? → 93Prompt: Find the third angle first, then use the straight line.

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.
  1. In a triangle, two angles are 30° and 70°. One side is extended to make an exterior angle next to the third angle. How big is that exterior angle? → 100Prompt: Find the third angle first, then use the straight line.
  2. The angles of a triangle are x, x and 2x. What is the largest angle, in degrees? → 90Prompt: They add to 180°: 4x = 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: 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

  • Move on to: Use the exterior angle of a triangle.
  • 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

eurekami

Missing angles in triangles, including isosceles and algebra

Name

Date

Score/ 6

  1. Question 1

    In a triangle, two angles are 50° and 54°. One side is extended to make an exterior angle next to the third angle. How big is that exterior angle?

    Answer

  2. Question 2

    The angles of a triangle are x, 2x and 3x. What is the largest angle, in degrees?

    Answer

  3. Question 3

    An isosceles triangle has a top angle of 54°. The other two angles are equal. What is each of them?

    ??54°

    Answer

  4. Question 4

    In a triangle, two angles are 37° and 63°. One side is extended to make an exterior angle next to the third angle. How big is that exterior angle?

    Answer

  5. Question 5

    The angles of a triangle are 2x, 2x and 5x. What is the largest angle, in degrees?

    Answer

  6. Question 6

    An isosceles triangle has two equal base angles of 78°. What is the third angle?

    78°78°?

    Answer

Free from eurekami.com

for the adult only

Answer key: Missing angles in triangles, including isosceles and algebra

  1. 1. 104

  2. 2. 90

  3. 3. 63

  4. 4. 100

  5. 5. 100

  6. 6. 24