octogon
sides=4
draw lines from the outside angles to the center so they form 8 equal triangle sections
find the area of each triangle and multiply by 8
area of triangle=1/2 base times height
base=4
the height is the distance from the center of one side of the octogon, to the center=5
so area=4 times 1/2 times 5=4 times 5=10
8 triangles so 8 times 10=80
the answer is 80cm^2
the diagram included below shows area for a pentagon
