For the x-y plane region meeting last condition:
using Plots;gr()
f(x,y) = x + y - 2
g(x,y) = x - y - 1
θ(x,y) = (f(x,y) < 10) & (g(x,y) < 20 ) ? 1 : NaN
x = y = LinRange(-30,30,1000)
heatmap(x,y,θ, c=:lightblue, legend=false, ratio=1, xlims=extrema(x),ylims=extrema(y))
This other way displays the boundary too:
using Colors, Plots;gr()
f(x,y) = x + y - 2
g(x,y) = x - y - 1
θ(x,y) = (f(x,y) < 10) & (g(x,y) < 20 ) ? 1 : -1
x = y = LinRange(-30,30,1000)
Plots.contourf(x,y,θ, c = palette([:white,:lightblue]), legend=false,ratio=1, xlims=extrema(x),ylims=extrema(y))
Plots.contour!(x,y,θ, c=:red, lw=2,levels=[0])
