4.1.1 ANGLES IN A STRAIGHT LINE
Key Rule:
Angles in a straight line add up to 180°.
This means when angles lie on a straight line, their total sum must equal 180 degrees.
4.1.2 ANGLES AT A POINT
Key Rule:
Angles around a point describe the sum of angles that form a full turn.
Angles at a point add up to 360°.
Example 4.3
Calculate the value of angle a.
Summary of Key Facts
| Concept | Rule |
|---|---|
| Angles in a straight line | Add up to 180° |
| Angles at a point | Add up to 360° |
Exercise
(a)
Find the value of x.
60° + 87° + x = 180°
147° + x = 180°
x = 180° − 147°
x = 33°
(b)
Find the value of x.
x + 3x = 180°
4x = 180°
x = 45°
(c)
Find the value of x.
75° + 45° + 83° + x = 360°
203° + x = 360°
x = 157°
(d)
Find the value of x.
x + 3x + 5x = 360°
9x = 360°
x = 40°