All you need to do is show two things. In a regular polygon, all angles are congruent. That means all their measures are the same. Also, in any polygon, the sum of the measures of the interior angles of a polygon is (n - 2)180, where n = number of sides. Since a pentagon has 5 sides, the sum of measures of the interior angles is (5 - 2)180 = 3(180) = 540 Since all angles are congruent, each angle is 540/5
