> # QUADRIC SURFACES (section 11.6)      Math II, Fall 1996, 9/9/96
> #
> # Consider the curve
> #
> #      x**2 + y**2 - z**2 = 1    (1)
> #
> #
> #  We can parametrize this as follows:
> #
> x := cos(t)*cosh(s); y:= sin(t)*cosh(s); z := sinh(s);

                               x := cos(t) cosh(s)

                               y := sin(t) cosh(s)

                                  z := sinh(s)
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> #
> #  because   cos(t)**2 + sin(t)**2 = 1 for any t
> #
> #  and       cosh(s)**2 - sinh(s)**2 = 1 for any s
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> #
> #  so with any choice of t, we get
> #
> #     x**2 + y**2 = cosh(s)**2,
> #
> #  so
> #
> #     x**2 + y**2 - z**2 = 1 for any s and t.
> #
> #  Conversely, any point that satisfies (1) can be represented
> #  by this parametrization.
> #
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> #  We can plot the surface as follows:
> #
> plot3d([x,y,z],s=-2..2,t=0..2*Pi,title=`Hyperboloid`);
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>