what is the sum of the interior angles of a triangle
In the diagram shown above, we see triangle ABC and the dotted line through A that is parallel to BC. Since angle A, from the triangle, and angles 1 and 2 form a straight (dotted) line, the sum of their measures is 180Âº. We also know that since the dotted line is parallel to BC that by alternate interior angles. Theorem: The sum of the three angles in a triangle is always 180Âº.