The interior angles of a triangle sum up to 180 degrees.[br]The interior angles of a quadrilateral sum up to 360 degrees.
The interior angles of a pentagon sum up to 540 degrees.[br]The interior angles of a hexagon sum up to 720 degrees.
The interior angles of a heptagon sum up to 899.99 or 900 degrees.[br]The interior angles of an octagon sum up to 1080 degrees.
In the pentagon the diagonal create 3 triangles, all equal to 180. As to show [math]180\times3[/math] will give you the total measurements of all the interior angles summed up in a pentagon.