A cylinder's side unrolls into a rectangle and a cone's into a sector, and the arc length of that sector traces back to the base's circumference.
Module goal. Use nets to flatten a solid's surface into a plane figure, turning surface-area problems into the flat-area calculations you already know.
eastmath.com · 28.3 Unfolding Surfaces and Surface Area · 28.3.2 Unfolding Prisms, Cones, and Frustums