Drawings of Number of Degrees in a Polygon

There are two less triangles in each shape than the number of sides
A triangle, is well, a triangle so 180 degrees
A square has two triangles in it so 360
A pentagon has three triangles in it so 540
The hexagon above was cut into two trapezoids so 2 x 360 = 720
See  the other post "Number of Degrees in a Polygon"