Construct a rectangle and draw a diagonal separating it into two identical triangles.  We now have two angle measures (both 90 degrees) of the rectangle. Then, we can prove alternate interior angles equal and since there are two pairs, adding them up will result in 180 degrees. Therefore, the rectangle has a total interior angle measurement of 360 degrees. Since there are two identical triangles, we can divide this by 2 to obtain 180 degrees.