Best Answer

The sum of the exterior angle and interior angle is 180°:

→ interior_angle + exterior_angle = 180

→ exterior_angle = 180° - interior_angle

In an equilateral triangle, each interior angle is 180° ÷ 3 = 60°

→ exterior_angle = 180° - 60° = 120°

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Alternatively:

The sum of the exterior angles of a polygon is 360°

For a regular polygon they are all the same size:

→ number_of_sides × exterior_angle = 360°

→ exterior_angle = 360° ÷ number_of_sides

An equilateral triangle is a regular polygon with 3 sides:

→ exterior_angle = 360° ÷ 3 = 120°

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Q: What is the size of an exterior angle in an equilateral triangle?

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