### Video Transcript

What is the size of the exterior angle of an equilateral triangle?

An equilateral triangle has three equal sides. This means that it must also have three equal interior angles. The sum of the angles in any triangle is equal to 180 degrees. Therefore, we can calculate each interior angle by dividing 180 by three. 180 divided by three is equal to 60. Therefore, each interior angle of an equilateral triangle is equal to 60 degrees.

The interior and exterior angle of any polygon add up to 180 degrees. In the case of the equilateral triangle, 𝑥, the exterior angle, plus 60 equals 180. Subtracting 60 from both sides of this equation gives us a value of 𝑥 equal to 120. This means that the exterior angle of an equilateral triangle is equal to 120 degrees.

The sum of all the exterior angles is always 360 degrees. So we could also have calculated this by dividing 360 by three.