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Bug in Plot3D?

• To: mathgroup at smc.vnet.net
• Subject: [mg105217] Bug in Plot3D?
• From: SixWingedSeraph <wellsoberlin at gmail.com>
• Date: Tue, 24 Nov 2009 05:51:44 -0500 (EST)

The two plot commands in the notebook below should give identical
outputs, but they don't.  The notebook is posted at http://abstractmath.org/MM/MmaBugQ.nb

a := 9; b := 18;

ff[x_, y_, z_] := x + y + z

gg[x_, y_, z_] := 2*x - 2*y - z

f[x_, y_] := z /. Solve[ff[x, y, z] == a, z]

f[x, y]

g[x_, y_] := z /. Solve[gg[x, y, z] == b, z]

g[x, y]

Plot3D[{-x - y + 9, 2*(x - y - 9)}, {x, 6, 10}, {y, -5, 1},
   BoxRatios -> {4, 6, 24}, ViewPoint -> {4.5, 3, 20},
 MeshStyle -> Gray,
   AxesLabel -> {x, y, z}, PlotStyle -> {{Red, Opacity[0.7]},
       {Green, Opacity[0.7]}}, ColorFunction -> White]

Plot3D[{f[x, y], g[x, y]}, {x, 6, 10}, {y, -5, 1},
   BoxRatios -> {4, 6, 24}, ViewPoint -> {4.5, 3, 20},
 MeshStyle -> Gray,
   AxesLabel -> {x, y, z}, PlotStyle -> {{Red, Opacity[0.7]},
       {Green, Opacity[0.7]}}, ColorFunction -> White]