Use the formula for the area of a circle to show that doubling the diameter of a pizza quadruples its area. The formula for the area of a circle of radius $r$ is $A=$ Enter “pi” to get $\pi\text{.}$ If we double the diameter, the new radius is Substitute this expression into the area formula to get the area of the new circle $A_\text{new}=$ So $A_\text{new}$ is times the original area.