Year 7 · Maths · Lines and angles · Lesson 3
Angles in triangles and quadrilaterals
Use the angle sums of triangles and quadrilaterals, including isosceles and equilateral triangles, to find missing angles.
25-minute video lesson
Teaching, worked examples and 20 practice questions solved step by step.
Watch free for a monthWhat your child will learn
- Know angles in a triangle add to 180°
- Know angles in a quadrilateral add to 360°
- Use isosceles triangle properties
- Find missing angles
- Explain angle facts
The method
- Triangle: 180° minus the other two angles.
- Quadrilateral: 360° minus the other three.
- Isosceles: look for the two equal angles.
- Give the reason.
Key words: triangle, quadrilateral, isosceles, equilateral, angle sum.
Worked examples
The first three practice questions from the lesson, with every step.
Question 1
Find the size of angle x.
- Step 1: Look at the triangle
- Step 2: Angles in a triangle add up to 180°.
- Step 3: x = 180° − 60° − 80° = 40°
Answer: x = 40°
Question 2
Find the size of angle x.
- Step 1: Look at the triangle
- Step 2: Angles in a triangle add up to 180°.
- Step 3: x = 180° − 45° − 45° = 90°
Answer: x = 90°
Question 3
Find the size of angle x.
- Step 1: Look at the triangle
- Step 2: Angles in a triangle add up to 180°.
- Step 3: x = 180° − 30° − 100° = 50°
Answer: x = 50°