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Volumes of Solids

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Volumes of Solids

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Volumes of Solids

Volumes of Solids

Volumes of Solids

The application of integrals to the computation of areas in the plane can be extended to the computation of certain volumes in space, namely those of solids of revolution. A solid of revolution arises from revolving the region below the graph of a function f (x) about the x - or y -axis of the plane. A cone arises in this way from a triangular region, a sphere from a semicircular region, and a cylinder from a rectangular region. These are just a few of the possibilities for solids of revolution.

There are two primary methods for finding the volume of a solid of revolution. The shell method is applied to a solid obtained by revolving the region below the graph of a function f (x) from a to b about the y -axis. It approximates the solid with a number of thin cylindrical shells, obtained by revolving about the y -axis the thin rectangular regions used to approximate the corresponding region in the plane. This is illustrated in the figure below.

Figure %: The Shell Method of Finding the Volume of a Solid of Revolution

The volume of a thin cylindrical shell of radius x , thickness Δx , and height f (x) is equal to


Π(x + )2 f (x) - Π(x - )2 f (x) = Π(2xΔx)f (x)  
  = (2Πx)(Δxf (x))  

Here by "cylindrical shell" we mean the region between two concentric cylinders whose radii differ only very slightly; precisely speaking, this formula is not correct for any positive thickness, but approaches the correct value as the thickness Δx shrinks to zero. Since we will ultimately consider such a limit, this formula will yield the correct volume in our application.

If we sum together the volumes of a family of such cylindrical shells, covering the entire interval from a to b , and take the limit as Δx→ 0 (and consequently as the number of cylindrical shells approaches infinity), we end up with the integral

Vol = 2Πxf (x)dx = 2Π xf (x)dx    

The disk method for finding volumes applies to a solid obtained by revolving the region below the graph of a function f (x) from a to b about the x -axis. Here the solid is approximated by a number of very thin disks, standing sideways with the x -axis through their centers. These disks are obtained by revolving about the x -axis the thin rectangular regions used to approximate the area of the corresponding region in the plane. This is illustrated in the figure below.

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