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Triangle Angles 2



To review the basics of triangle angles, see here.

In this topic, you will need to understand supplementary angles. Two angles are supplementary angles if the sum of their angles is 180o. Angles on a straight line always add up to 180o.
This example shows angles on a straight line.
The angles 65o and 115o are supplementary angles because their angles add up to 180o.


Example 1: Supplementary angles

Find the value of 'x'. Diagrams are not always to scale.
x =
The angles that measure 2x and 18x lie on a straight line so they are supplementary angles.
This means that 2x + 18x = 180.
2x + 18x = 180
20x = 180
20x ÷ 20 = 180 ÷ 20
x = 9


Example 2: Triangle angles

Find the value of 'x'. Diagrams are not always to scale.
x =

The angle next to the 79o angle and the 79o angle are angles on a straight line so they are supplementary angles.

angle next to 79o + 79o = 180o
angle next to 79o = 180o - 79o

Since the angles of a triangle add up to 180o, we know that
xo + 34o + angle next to 79o angle = 180o
x + 34 + (180 - 79) = 180
x + 34 + 101 = 180
x + 135 = 180
x = 45

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