# What is the area of a circle, if the diameter is 18?

We know that $A = {r}^{2} \pi$, and that $d = 2 r$. Thus
$18 = 2 r \to r = 9$
$A = {9}^{2} \pi = 81 \pi \approx 254.47 \text{ } {u}^{2}$