14.12.5  The Cartesian equation of an object in space: equation

See Section 13.13.7 for Cartesian equations of two-dimensional objects.

The equation command finds equations for geometric objects.

• equation takes one argument:
  G, a geometric object.
• equation(G) returns Cartesian equations in x, y and z which specify the object G.
  The variables x, y and z must be unassigned. If they have assignments, they can be unassigned with purge(x,y,z).

Examples.

• Input:
  equation(line(point(0,1,0),point(1,2,3)))
  Output:

  x−y+1=0,3 x+3 y−2 z−3=0

• Input:
  equation(sphere(point(0,1,0),2))
  Output:

  x2+y2−2 y+z2−3=0