       # Re: Mathematica Plot [help]

```
Wei Xu wrote:

> I want to draw a surface, say sin[x,y],  defined on a triangular domain,
> say, x:[0,1], y:[0,1] and x+y <=1.
> I tried to use the following command:
>  ParametricPlot3D[{fx, fy, fz}, {x,0,1}, {y,0,1-x}] but it is not
> allowed to use {y,0,1-x}.

Wei:
Find a parameterization of the region, Rxy, that you wish to plot f[x,y]
over.

{s,t} -> {x[s,t], y[s,t]}

taking a rectangle  [smin,smax] x [tmin, tmax] onto R[x,y].

Plot

ParametricPlot3D[{x[s,t],y[s,t],f[x[s,t],y[s,t]},
{s,smin,smax},{t,tmin,tmax}]

x[s_,t_] := s
y[s_,t_] :=  s t
f[x_,y_] := Sin[x y]

ParametricPlot3D[{x[s,t], y[s,t], f[x[s,t],y[s,t]]},{s,0,1}, {t,0,1}]

There are many ways of implementing this idea: here is one using
formulas and replacement.

{x,y,z}/.z->Sin[x y]/.{x->s, y-> s t}

{s, s*t, Sin[s^2*t]}

ParametricPlot3D[%//Evaluate,{s,0,1}, {t,0,1}]

This will work without the use of Evaluate, but less efficiently --
Allan Hayes
Mathematica Training and Consulting
Leicester, UK
hay@haystack.demon.co.uk
http://www.haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44 (0)116 271 8642

```

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