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This equation is for an elliptic cylinder. If a = b then the surface is a circular cylinder. The cylinder is a degenerate quadric because at least one of the coordinates (in this case z) does not appear in the equation. By some definitions the cylinder is not considered to be a quadric at all.
In common usage, a cylinder is taken to mean a finite section of a right circular cylinder with its ends closed to form two circular surfaces, as in the figure (right). If the cylinder has a radius r and length h, then its volume is given by
- V = πr2h
and its surface area is
- A = 2πr(r + h)
For a given volume, the cylinder with the smallest surface area has h = 2r. For a given surface area, the cylinder with the largest volume has h = 2r.
There are other more unusual types of cylinders. These are the imaginary elliptic cylinders:
the hyperbolic cylinder:
and the parabolic cylinder:
- x2 + 2y = 0
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