For the other polygons, we can figure out the sum of angles by
dividing the polygons into triangles. Any polygon can be separated
into triangles by drawing all the diagonals that can be drawn from
one single vertex.
In the quadrilateral shown below, we can draw only one diagonal
from vertex A to vertex B. So, a quadrilateral can be separated
into two triangles.
The sum of angles in a triangle is 180º. Since a quadrilateral
is made up of two triangles the sum of its angles would be 180º
× 2 = 360º