How do you find the area of a compact disc?

Answer 1

Suppose the diameter of the disk is D cm so that the radius is D/2 = R cm.

Suppose the diameter of the hole is d cm so that its radius is d/2 = r cm.

Then the area of one side of te disc is

pi*(R^2 - r^2)

Therefore the total area of the two sides is 2*pi*(R^2 - r^2) square cm.

This ignores the surface area of the edge which, although small, is not 0.

Answer 2

You measure its outer diameter, divide by 2 to give the outer radius = R units.You measure its inner diameter, divide by 2 to give the inner radius = r units [radius of the hole].

Then area of the annulus = pi*(R^2 - r^2) square units.