Find the length and width of a rectangle that has the given area and a maximum area. perimeter: 80 meters

IF THIS IS FOR 40 WHAT IS IT FOR 80

For maximum area, you've got a square. So 10 x 10. Proof 2x + 2y = 40 x + y = 20 Max xy =Max x(20-x) = Max 20x - x^2 Take 1st derivative and set to zero 20 - 2x = 0 2x = 20 x = 10 So y = 10

The maximum area of a shape will always be a square. So, the sides will all be 20, because four of those add up to 80.
Length and width will be 20.
That would make the area 400 units².

What is the inequality?