Lines and Angles
Primary 4 · Mathematics
Lines and Angles
Lesson 3 of 5
25 min
Let's begin
Gatete opens the classroom door a little, then a lot. "The door turned more the second time," says the teacher. "The opening between the door and the wall is an angle."
An angle tells you how much something has turned. Today you will see angles in doors, clocks and roofs.
An angle is made when two rays start from the same point. The two rays are the arms of the angle. The point where they meet is the vertex.
We name this angle angle ABC, and we write it ∠ABC. The vertex letter always goes in the middle. You may also call it ∠CBA, or simply ∠B.
Imagine one arm lying still while the other arm turns away from it. The more it turns, the bigger the angle. Making the arms longer does not make the angle bigger. A short door and a tall door that open the same amount make the same angle.
We measure angles in degrees, written with a small circle: 90°.
The hands of a clock make angles. There are 12 spaces between the numbers, and a full turn is 360°. So each space is 360 ÷ 12 = 30°.
At 3 o'clock the hands are 3 spaces apart: 3 × 30° = 90°, a right angle. At 6 o'clock they are 6 spaces apart: 6 × 30° = 180°, a straight line.
Tip
Vertex in the middle. In ∠ABC the middle letter, B, is the corner. And remember the clock: each number space is 30°.
Work each one in your exercise book first. Then tap Show answer to check.
In ∠PQR, which letter is the vertex?
Answer: Q, the middle letter.
How many degrees are in a half turn?
Answer: 180°.
The minute hand turns from 12 to 6. Is that a quarter, a half or a full turn?
Answer: A half turn, 180°.
What angle do the hands of a clock make at 2 o'clock?
Answer: 60°, because 2 × 30° = 60°.
Uwase draws an angle with arms 3 cm long. Byiringiro copies it with arms 10 cm long, opened by the same amount. Whose angle is bigger?
Answer: Neither. The angles are the same size, because the arms turned by the same amount.
What you learned today
Sources and credits
The stories, Rwandan examples, diagrams and practice questions were written by Ganzaa.
Quick check
In ∠KLM, which letter is the vertex?
