Remember that a triangle is 1/2 of a square. A square has 4 interior angles, which are all exactly 90 degrees (if they were not all 90 degrees, it would not be a square). 90 x 4 = 360. So if a square / 2 = a triangle, then the interior angles of a square / 2 = the interior angles of a triangle.
360 / 2 = 180.
