## Algebra and Trigonometry 10th Edition

$A=\frac{C^2}{4\pi}$
If the radius of the circle is r, then the circumference $C = 2\pi r$, giving $r=\frac{C}{2\pi}$, and the area is $A = \pi r^2=\pi (\frac{C}{2\pi})^2=\frac{C^2}{4\pi}$