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Year 7 Interactive Maths - Second Edition


Exterior Angle of a Triangle

An exterior angle of a triangle is formed when any side is extended outwards.

Angle ACD is the exterior angle of triangle ABC as the side BC is extended outwards to D.

ACD is an exterior angle of triangle ABC

Clearly, the exterior angle ACD and the adjacent interior angle ACB are supplementary.  That is:

Angle ACD + Angle ACB = 180°


Example 9

Find the value of the pronumerals in the following diagram.

Find the value of the pronumerals x and y.

Solution:

x = 122° since the sum of adjacent angles forming a straight line is 180°

y = 62° as the angle sum of a triangle is 180°


Key Term

exterior angle of a triangle

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