<Dl> <Dd> A = π x y . (\ displaystyle A = \ pi xy .) </Dd> </Dl> <Dd> A = π x y . (\ displaystyle A = \ pi xy .) </Dd> <P> Most basic formulas for surface area can be obtained by cutting surfaces and flattening them out . For example, if the side surface of a cylinder (or any prism) is cut lengthwise, the surface can be flattened out into a rectangle . Similarly, if a cut is made along the side of a cone, the side surface can be flattened out into a sector of a circle, and the resulting area computed . </P> <P> The formula for the surface area of a sphere is more difficult to derive: because a sphere has nonzero Gaussian curvature, it cannot be flattened out . The formula for the surface area of a sphere was first obtained by Archimedes in his work On the Sphere and Cylinder . The formula is: </P>

All measurements of circle a can be determined using which of the following