Present that the sum of the interior angles of a triangle is 180^{0} (You can prove that the interior angles add up to 180^{o} by tearing a paper triangle into three pieces and lining the three angles up to make a straight line).
Present the Triangle Sum Theorem (the acute angles of a right triangle are complementary).
Introduce the Exterior Angles Theorem (the measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent interior angles).
Provide examples that demonstrate how to apply these theorems and solve for unknown variables and unknown interior and exterior angles of a triangle.
This packet should help a learner seeking to understand the sum of the interior angles of a triangle and how to solve for triangle angle measures.
This video explains that the sum of a triangle's angles is 180 degrees.
This video presents another explanation of why the sum of the angles in a triangle is 180 degrees.
This video introduces the exterior angles theorem.