What is the exact area of a circle whose diameter is 18 cm? A. 9 straight pi cm2 B. 18 straight pi cm2 C. 36 straight pi cm2 D. 81 straight pi cm2

The formula for the area of a circle is:

A=πr²

We also know that the diameter of a circle is twice the size of its radius

d=2r

So if the diameter is 18 ...

18=2r

divide both sides by 2

9=r

The radius of the circle is equal to 9cm

Now we can solve for the area

A=πr²

A=π9²

A=π81

A=81πcm²

The exact area of a circle whose diameter is 18cm is 81πcm²

Answer=D