what is a triangles interior angle sum
its interior angles add up to 3 Ã 180Â° = 540Â°. And if it is a regular pentagon (all angles the same), then each angle is 540Â° / 5 = 108Â°. (Exercise: make sure each triangle here adds up to 180Â°, and check that the pentagon's interior angles add up to 540Â°) The Interior Angles of a Pentagon add up to 540Â°.