Enter here an algebraic expression that gives the area of a circle with a radius $r$. You may write "pi" to denote the symbol $\pi$, and you can use the symbol $\hat{}$ or a double asterisk ** to denote exponentiation. (Or you can express $r^2$ as $r$*$r$.)

Enter here an algebraic expression that gives the area of a circle with a diameter $d$.