Formulas for the area of a rectangle, other than $l \times w$.

Question

So you're asked to show two formulas for the area of a rectangle. They can't include the classic $A=l \times w$

This is all I have come up with $$A=\int_0^{l}wdx$$ It doesn't matter how complex they are. Any suggestions?

Answer 1

Let $p$ be the perimeter of the rectangle and $C$ the area of the circle which circumscribes the rectangle. Then $$ A=\frac{p^2}{8} - \frac{2C }{\pi}. $$