Can anyone find a formula for the described function A rectangle has a perimeter of 20m Express the area of the rectangle as a function of the length of one of its sides?

Suppose the length and width of the rectangle are L and W metres respectively.Then the perimeter, P = 20 m implies that
2(L + W ) = 20 => L + W = 10 or W = 10 - L.

Then Area = L * W = L * (10 - L) sq metres.