 Pappus's area theorem

Pappus's area theorem, also known as Pappus's centroid theorem, is a fundamental result in geometry concerning the surface area of a solid of revolution. The theorem states that the surface area \( A \) of a solid formed by revolving a plane figure about an external axis (that is not intersecting the figure) is equal to the product of the length of the path traced by the centroid of the figure and the area of the figure itself.