Regular octagon is a regular 8-gon (8 vertices, 8 sides). The exterior angle of the regular octagon is 360 degrees divided by 8, i.e.= 45 degrees. Twenty eight times 45 degrees is 1260 degrees. Thus the sum of interior angles of the polygon is 1260 degrees. Using formula for the sum of the interior angles of a n-gon, you have this equation (n-2)*180 = 1260 degrees, or n - 2 = 1260/180 = 7. Hence, n = 7+2 = 9, and the polygon is 9-gon (nonagon). ANSWER