Answer
$s=\frac{\sqrt 2}{2}d$
$a=\frac{1}{2}d^2$
$s$ - side
$d$ - diagonal
$a$ - area
Work Step by Step
We'll use s for side, d for diagonal and a for area.
We know $s^2+s^2=d^2$, which makes $s^2=\frac{d^2}{2}$
Since $s^2=\frac{d^2}{2}$ and $s^2=a$, we can conclude that $a=\frac{d^2}{2}$
