In the derivation of the formula for volume of a sphere, we started with a cylinder with two cones inside. Imagine passing a horizontal plane through the figure, creating radius b, as shown below.
What is the cross-sectional area of the shaded ring?
πb2 – πr2, or π(b2 – r2)
πb2 + πr2, or π(b2 + r2)
πr2 – πb2, or π(r2 – b2)
πr2 + πb2, or 2π(r2 + b2)