Most Math 2 plane geometry calculations are about closed figures: polygons and circles. And the test
makers’ favorite closed figure by far is the three-sided polygon; that is, the triangle. All three-sided
polygons are interesting because they share so many characteristics, and certain special three-sided
polygons—equilateral, isosceles, and right triangles—are interesting because of their special
characteristics.
Let’s look at the traits that all triangles share.
Sum of the interior angles: The three interior angles of any triangle add up to 180°.
In the figure above, x + 50 + 100 = 180, so x = 30.
Measure of an exterior angle: The measure of an exterior angle of a triangle is equal to the sum of
the measures of the remote interior angles.
In the figure above, the measure of the exterior angle labeled y° is equal to the sum of the measures
of the remote interior angles: y = 50 + 100 = 150.
Sum of the exterior angles: The measures of the three exterior angles of any triangle add up to
360°.