Question

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

Answer 1

`81pi`

Explanation:

The area `A` of a circle with radius `r` is given by `A=pir^2`

The radius of a circle is half its diameter. That is, `r = d/2`

Putting these together, if `d = 18` we have

`A = pir^2 = pi(18/2)^2 = pi9^2 = 81pi`