Actually, people made formulas for polygons, but in another way, the definition of the perimeter is the sum of the lengths of all sides. Example:
Rectangle:
It has length and width right? so you can write it as:
P=L+L+W+W
or simply
P=2L+2W
or if you factor it out;
P=2(L+W)
Square:
It has all sides equal right?
P=s+s+s+s
P=4s
Triangle:
sometimes it doesn't have equal sides
P=a+b+c
but if it is equal
P=3s
Hope this helps =)
