We know that each angle is 720/n
Formula for each interior angle of polygon is 180(n-2)/n
The number of sides in the polygon are "m", so sub in m into the formula:
180(m-2)/m which is equivalent to 720/n each angle in the polygon
:. 180(m-2)/m = 720/n
divide both sides by 180
(m-2)/m=4/n
cross multiply
(m-n)n=4m
expand
mn-2n=4m
mn-4m=2n
factor out the "m"
m(n-4)=2n
:. m = 2n/(n-4)