The area formula for regular polygons =
1/2 apothem x perimeter
the apothem is the perpendicular distance from the center of the polygon to a side
the formula really adds the areas of the eight triangles formed by connecting the center to the 8 vertices. the apothem is the height of each triangle.
you might need trig to find some of the values
an interior angle of a regular octagon is 135 degrees
if you don't have a regular octagon, you'll have to divide it up into triangles
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