Angle Sum Property
You will very likely be already familiar with the angle sum property of triangles. In any triangle, the sum of the three angles in 1800. This is true regardless of what kind of triangle we have. The following diagram shows some examples of different kinds of triangles. In all cases, note that the sum of the three angles is exactly 1800.
As an exercise, you should randomly construct three triangles. Then, for each triangle, measure the three angles using a protractor, and verify that their sum is indeed 1800 (to reasonable accuracy).