Multiply π by the diameter of the circle. The formula using the radius is 𝑪 = 2π𝒓. This means that 2 is multiplied by π and then multiplied by the radius.

If I cut out circles of different diameters and measure the mass of the circle then I can calculate the area. If the area is also supposed to be π R 2 , I can make a plot of area vs. diameter ...