The circumference of a circle (C) is described by the formula:
[tex]C=2\pi r[/tex]
Where r is the radius of the circle. We can solve for r in this equation to calculate its value, like this:
[tex]\begin{gathered} C=2\pi r \\ \frac{C}{2\pi}=\frac{2\pi r}{2\pi} \\ \frac{C}{2\pi}=r\times\frac{2\pi}{2\pi} \\ r=\frac{C}{2\pi} \end{gathered}[/tex]
Replacing the value of the circumference, we get:
[tex]r=\frac{18\pi}{2\pi}=\frac{18}{2}=9[/tex]
Then, the radius of the circle is 9 feet
