Pappus's centroid theorem

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Pappus's centroid theorem (also known as the Guldinus theorem, Pappus-Guldinus theorem or Pappu's theorem) is the name of two related theorems dealing with the surface areas and volumes of surfaces and solids of revolution.

The theorem is attributed to Pappus of Alexandria and Paul Guldin.

The first theorem

The first theorem states that the surface area A of a surface of revolution generated by rotating a plane curve C about an axis external to C and on the same plane is equal to the product of the arc length s of C and the distance d₁ traveled by its centroid.

[A = sd_1.\,]

For example, the surface area of the torus with minor radius r and major radius R is

[A = (2\pi r)(2\pi R) = 4\pi^2 R r.\,]

The second theorem

The second theorem states that the volume V of a solid of revolution generated by rotating a plane figure F about an external axis is equal to the product of the area A of F and the distance d₂ traveled by its geometric centroid.

[V = Ad_2.\,]

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