Our formula for the area of a circle is
A=(pi)r2
Also, radius is half of the diameter. So if the diameter is 18, then the radius is 9.
A=(pi)(9) 2
A=(pi)(81)
$${\mathtt{A}} = \left({\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{81}}\right) \Rightarrow {\mathtt{A}} = {\mathtt{254.469\: \!004\: \!940\: \!773\: \!252\: \!3}}$$
So about 254.5 inches 2 rounded to the neathest tenth.
If you wanted to do this without a calculator you could estimate pi to 3.14. You could do this because we only need an approximate area of the circle.
Regaurdless, you will get somewhere around 254 inches2.
+3454 
