## Precalculus (6th Edition) Blitzer

$r=\dfrac{C}{2\pi}$ This formula gives the radius of a circle where C = circumference of the circle.
Divide $2\pi$ on both sides of the equation to obtain: $\require{cancel} \dfrac{C}{2\pi} = \dfrac{2\pi{r}}{2\pi} \\\dfrac{C}{2\pi} = r \\r=\dfrac{C}{2\pi}$ This formula gives the radius of a circle where C = circumference of the circle.