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This angle is also a right angle. Angles on a Line 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.

Angles on a straight line add to 180o 90 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

Find the unknown angles x 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