What is the sum of all measures of the interior angles of a regular three-sided polygon?

a. 100°

b. 150°

c. 180°

d. 360°

The sum of the three angles inside a 3-sided polygon ...

commonly referred to as a "triangle" ... is 180 degrees.

It doesn't matter whether the polygon is regular or not.

Any three sides make three angles that total 180 degrees.

The sum of the interior angles of a polygon is:

180 (n-2)

Where n is the number of sides. Therefore

180 (1)

=180

So the interior angles add up to 180 degrees in a triangle.