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I have two functions $f(x,y)$ and $g(x,y)$. I wish to perform two tasks on a 2-dimensional figure:

  1. Plot the curve where each point satisfies $f(x,y)=g(x,y)$.

  2. Mark one side of the curve with red.

I tried to use Plot3D like the following

myfunc[x_, y_]:= If[f[x, y] < g(x, y), 1,0];
Plot3D[myfunc[x, y], {x, 0, 10}, {y, 0, 3}]

It gives what I expected, but how can I do this in a 2D plot?

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RegionPlot[f[x, y] <= g[x, y], {x, 0, 10}, {y, 0, 3}]
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  • $\begingroup$ thanks! I didn't know this before. $\endgroup$ – David Xiaoyu Xu Mar 20 at 23:41
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ContourPlot[f[x, y] - g[x, y], {x, 0, 10}, {y, 0, 3}, 
  Contours -> {{0}}, ContourShading -> {None, Red}]
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