dangerousliri 09.01.2017 18:36 In all rectangles with same diagonal $d$ find that one with bigger area .
doitsudoitsu 09.01.2017 18:44 Click to reveal hidden text Let a and b be the sides of the rectangle. $a^2+b^2=d^2$, and we wish to maximize $ab$. By AM-GM, $\frac{(a+b)^2}{4} \geq ab$ and we wish to maximize $a+b$. This happens when $a=b$, or when the rectangle is a square.