A certain coin is a circle with diameter 18 mm. What is the exact area of either face of the coin?

The correct answer is 81.

Step-by-step explanation:

To find the area of the coin/a circle use this equation:

(a = area, r = radius, d = diameter)

a = r²

So we need to do for the radius.

r = d/2

r = 18/2

r = 9

Then solve

a = 9²

a = 81