Question

Why doesn’t this work?
I have bypassed this before but i can’t remember how i did it, and I never went on to figure out why this type of inputs didn’t work. About time to get to know it!

For those who cant see the pic:

RegionPlot3D[
 x^2 + 2 y^2 - 2 z^2 = 1 && -1 <= z <= 1, {x, -5, 5}, {y, -5, 
  5}, {z, -1, 1}]    
Set::write: "Tag Plus in -2.+25.+50. is Protected"

And then there is just an empty cube without my surface.

Answer 1

If z is limited by other surfaces you could go like this:

RegionPlot3D[
 x^2 + 2 y^2 - 2 z^2 < 1 && z < x + 2 y && z^2 < .5, 
 {x, -2, 2}, {y, -2, 2}, {z, -1, 1}, 
 PlotPoints -> 50, MeshFunctions -> {Function[{x, y, z}, z]}, 
 PlotStyle -> Directive[Red, Opacity[0.8]]]  

Or with ContourPlot:

ContourPlot3D[
 x^2 + 2 y^2 - 2 z^2 == 1,
 {x, -2, 2}, {y, -2, 2}, {z, -1, 1}, 
 RegionFunction -> Function[{x, y, z}, z < x + 2 y && z^2 < .5], 
 PlotPoints -> 50, MeshFunctions -> {Function[{x, y, z}, z]}, 
 ContourStyle -> Directive[Red, Opacity[0.8]]]]