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Strategy: You will have to figure out the area of a 12” pizza vs. the area of a 16” pizza. The formula for the area of a circle is Pi times the square of the radius.
The radius of a pizza is onehalf the diameter. If you enter the diameter of the pizza in B2, the radius is =B2/2.
Pi is a Greek letter that represents 3.141592654. Excel offers the PI function to return this number, as shown in Fig. 445. It is a lot easier to remember =PI() instead of 3.141592654. (Click any figure below for a larger view of that image.)

Set up a worksheet. In cell B2, enter the diameter of the pizza.

In cell C2, calculate the radius as =B2/2, as shown in Fig. 446.

In cell D2, calculate the area of the pizza in square inches, using =PI()*C2^2, as shown in Fig. 447.

In column E, enter the quantity of pizzas.

Calculate the total square inches in column F using =E2*D2, as shown in Fig. 448.

Enter the cost for the special in column G. In column H, calculate the pennies per square inch of pizza using =G2/F2, as shown in Fig. 449.
Result: From a purely mathematical point of view, the special with two large pizzas is a slightly better deal, pricing the pizza at 3.98 cents per square inch.
Additional Information: My seventh grade math teacher, Mr. Irwin, would like me to mention, for the sake of completeness, that the circumference of the pizza is Pi times the diameter. That would be =PI()*B2, as shown in Fig. 450.
Summary: To calculate the area of a circle, use the PI function times the radius squared.
Functions Discussed: =PI()