Lamar wanted to explain why the measure of angle 5 is equal to the sum of the measures of angles 1 and 3. A triangle with angles 1, 2, 3. The exterior angle to angle 1 is 4, to 2 is 5, to 3 is 6. Which geometry concepts should be used in Lamar’s work? supplementary angles and the sum of the measures of interior angles of a triangle complementary angles and the sum of the measures of interior angles of a triangle supplementary angles and vertical angles the sum of the measures of interior angles of a triangle and vertical angles