You already know that the interior angles of a triangle add up to 180º.
Polygons with more sides have a larger sum of interior angles.
To work out the sum of the interior angles of a polygon with more sides, we choose one of the corners and split the polygon into triangles. Each triangle will contain 180º.
Work out the sum of the interior angles of this hexagon.
Choose any one corner.
Draw diagonals from the chosen corner to form triangles.
There are 4 triangles inside the hexagon.
4 x 180º = 720º
The sum of the interior angles of a hexagon is 720º.