Definition: The area of a circle is pi times the square of its radius, A = pi r<sup>2</sup>, measured in square units.
What is the formula?
A = pi x r x r. The radius is the distance from the center to the edge.
Why does it work?
Cut a circle into thin wedges and rearrange them into a rough parallelogram. Its base is half the circumference, pi r, and its height is r, so the area is pi r times r.
How do you use it?
A pizza has a diameter of 16 inches, so r = 8. Then A = 3.14 x 82 = 3.14 x 64 = 200.96 square inches. A 12 inch pizza has r = 6 and A = 3.14 x 36 = 113.04.
Which one is the better value?
The 16 inch pizza has about 1.78 times the area of the 12 inch one, although its diameter is only 1.33 times larger. Area grows with the square of the radius.
What are the common mistakes?
Using the diameter in place of the radius, and forgetting to square only the radius. Writing (pi r)2 is wrong. First square r, then multiply by pi.