3D parametric surface: plotparam paramplot DrawParm

### 8.14.2 3D parametric surface: plotparam paramplot DrawParm

To draw a parametric surface in ℝ3:

- plotparam takes two mandatory arguments and two optional arguments:
  - exprs, a list of three expressions involving two parameters.
  - vars, a list of the parameters, optionally with a range var=a..b.
  - Optionally, ustep=n, to set the discretization step of the first parameter.
  - Optionally, vstep=m, to set the discretization step of the second parameter.
- plotparam(exprs,vars ⟨ ,ustep=n,vstep=m⟩) draws the parametric representation of the surface.

Examples.

- Input:
  plotparam([v*cos(u),v*sin(u),v],[u,v])
  Output:
- Input:
  plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3])
  Output:
- Input:
  plotparam([v*cos(u),v*sin(u),v],[u=0..pi,v=0..3],ustep=0.5,vstep=0.5)
  Output: