Learning Objectives

• Know what an angle is and name its parts (vertex and arms).
• Recognise and draw common types of angles: acute, right, obtuse and straight.
• Measure angles using a protractor and write angles in degrees (°).
• Understand complementary and supplementary angles with simple examples.

What is an angle?

An angle is made when two rays (lines with a starting point) meet at a point called the vertex. The two rays are called the arms of the angle. We measure angles in degrees (°).

Types of angles

Measuring angles with a protractor

1. Place the protractor so its centre hole is on the vertex of the angle.
2. Make sure one arm lines up with the zero line (baseline) of the protractor.
3. Read the number on the protractor where the other arm points. That number is the angle in degrees (°).

Complementary and supplementary

- Complementary angles: Two angles that add up to 90°. Example: 30° and 60° are complementary (30° + 60° = 90°).
- Supplementary angles: Two angles that add up to 180°. Example: 70° and 110° are supplementary (70° + 110° = 180°).

Practice questions

1. Draw an acute angle of 40° and a right angle. Label the vertex.
2. Which type is an angle of 130°? (Acute, right, obtuse or straight?)
3. If one angle is 35°, what is its complementary angle?
4. If two angles are supplementary and one is 125°, what is the other angle?
5. Measure the angle shown: (use the small diagram below)

Answers to practice

1. Draw a 40° acute angle and a 90° right angle. (Teacher checks drawing and labels.)
2. 130° is an obtuse angle.
3. Complementary angle: 90° − 35° = 55°.
4. Supplementary: 180° − 125° = 55°.
5. The picture shows an angle of 50°.