Write the circumference $C$ of a circle as a function of its area $A$.
In Example 9.5 the radius $r$ of a circle was expressed as a function of its area $A: r=\sqrt{\frac{A}{\pi}}$. Since $C=2 \pi r$, it follows that $\mathrm{C}=2 \pi \sqrt{\frac{A}{\pi}}$ expresses $C$ as a function of $A$.