A rectangle has perimeter equal to 40cm. What is the maximum possible area of this rectangle?

Question

A triangle, a square, a pentagon, a hexagon, an octagon and a circle all have an equal perimeter, which one has the smallest area?

Answer 1

100cm2
i.e
40 = 2(l+w)
l+w =20
l=20-w...........(1)
A = lw
= (20-w)w
=20w-w2
dA/dw =20 -2w
but at maximum A, dA/dw = 0
so 20-2w=0

w=10
which means l=20-10=10
so Area = 10*10=100cm2

Answer 2

10 * 10 is a square !
not a Rectangle.