The formula for the common measure of each of the interior angles of a regular polygon with n-sides is 145°.We set that equal to 145°: Multiply both sides by n Add 360° to both sides: Subtract 145°n to both sides Divide both sides by 35° Cancel the degrees: A regular polygon can only have a integer for its number of sides. Proved! ------------------------ Indirect proof: Assume that there is a regular polygon such that each of its interior angles of a regular polygon with n-sides is. The formula for the common measure of each of the interior angles of a regular polygon with n-sides is 145°. We set that equal to 145°: Multiply both sides by n Add 360° to both sides: Subtract 145°n to both sides Divide both sides by 35° Cancel the degrees: n is not an integer, so there is no such regular polygon. This contradicts the assumption that there is such a regular polygon. Proved! Edwin