1. This angle is also a right angle.
When a vertical line and a horizontal line meet the angle between them is 90o , a right angle.
90o, a right angle
Vertical line
This angle is also a right angle. 90
Horizontal line
This explains why the angles on a straight line add to 180o.
The lines are said to be perpendicular to each other.

2. Angles on a straight line add to 180o90
Angles on a straight line add to 180o
180o
Oblique line
Horizontal line a b
Angles a + b = 180o
70o b x
35o
Angle b = 180 – 70 = 110o
Angle x = 180 – 35 = 145o

3. Find the unknown anglesx
47o
Find the unknown angles 1
114o y 2 3 4
76o a
82o b
Angle x = 180 – 47 = 133o
Angle y = 180 – 114 = 66o
Angle a = 180 – 76 = 104o
Angle b = 180 – 82 = 98o