<Li> The angle sum of a triangle is greater than 180 ° and less than 540 ° . </Li> <Li> The area of a triangle is proportional to the excess of its angle sum over 180 ° . </Li> <Li> Two triangles with the same angle sum are equal in area . </Li> <Li> There is an upper bound for the area of triangles . </Li>

When do the special angle pairs have properties that work