Lines and Angles
Primary 4 · Mathematics
Lines and Angles
Lesson 4 of 5
30 min
Let's begin
Habimana's father is building a roof for the family's new kitchen. "The roof must slope at the right angle, or rain will stay on top," he says. He measures it with a tool shaped like half a circle.
That tool is a protractor. Today you will use one to measure and draw angles.
A protractor is half a circle marked in degrees from 0° to 180°. It has:
Here arm OA lies on the 0 of the blue outer scale. Follow the blue numbers up from 0: 10, 20, 30, 40, 50. Arm OB crosses at 50. So ∠AOB = 50°.
The orange inner scale says 130 at the same place. That is the reading from the other 0, so it is not our angle.
Before you read the scale, look at the angle. Is it smaller than a square corner? Then it is less than 90°. ∠AOB is clearly smaller than a right angle, so 50° makes sense and 130° cannot be right.
Tip
Centre on the corner, zero on the arm. Then count up from that zero. Always guess first: small corner, small number; wide corner, big number.
Work each one in your exercise book first. Then tap Show answer to check.
Where must the centre point of the protractor go?
Answer: Exactly on the vertex of the angle.
One arm lies on the 0 of the inner scale. The other arm crosses 35 on the inner scale and 145 on the outer scale. What does the angle measure?
Answer: 35°. Read the scale that starts at 0 on your arm, which is the inner scale. Check: 35° should look much smaller than a square corner.
An angle looks a bit smaller than a square corner. The two scales show 80 and 100. Which is the answer?
Answer: 80°, because the angle is less than 90°.
An angle is exactly a square corner. What does the protractor show?
Answer: 90° on both scales.
Describe how to draw an angle of 120°.
Answer: Draw an arm, put the centre on the vertex and the base line on the arm, count from that 0 up to 120, mark a dot and join it to the vertex.
What you learned today
Sources and credits
The stories, Rwandan examples, diagrams and practice questions were written by Ganzaa.
Quick check
Put the steps for measuring an angle in order.
Tap the cards in the right order.
