A rectangle has perimeter 40 cm. What is its largest possible area, in cm²?

Explanation

1. 01

   Sides $x$ and $20 - x$; area $A = x(20 - x)$.

2. 02

   $A' = 20 - 2x = 0$ at $x = 10$: the rectangle is a square, area $10^2 = 100$.