How to fill a specific region in a plot?

Question

I am having trouble filling a particular region in a plot.

I am trying to use Mathematica to plot the same image I linked below, and this is ok (except for all the numbers/letters labelling but this is not important).

What I have trouble with, is how to fill the region below the $x$-axis but above the green curve.

Again, I would also know how instead fill the entire region above the green curve, but under the parabola. I tried many combinations of the filling code but no results.

Thank you!

Answer 1

One way is using RegionPlot to add such region.

Clear[f, g, plot, reg];
f[x_] = 4 + 1/16 (-9 - 4 Sqrt[5]) (-19 + 8 Sqrt[5] + x)^2;
g[x_] = Log[5, x + 2] - 1;
plot = Plot[{f[x], g[x]}, {x, -2.5, 4.5}, 
   PlotStyle -> {Darker@Cyan, Darker@Green}, AspectRatio -> Automatic];
reg = RegionPlot[{g[x] <= y <= f[x] && y <= 0}, {x, -2, 3}, {y, -2, 
    0}, PlotStyle -> LightRed, BoundaryStyle -> None];
Show[reg, plot, PlotRange -> {-4, 4}, AxesStyle -> Arrowheads[{0.05}],
  Axes -> True, Frame -> False, 
 GridLines -> {{{-2, {Thick, AbsoluteDashing[{3, 3}]}}}, None}, 
 Epilog -> {{AbsoluteDashing[3, 3], Line[{{-1, -1}, {-1, 0}}], 
    Line[{{-1, -1}, {0, -1}}]}}]

Answer 2

Clear["Global`*"]

f1[x_] = -1 (2 x - 3) (x + 1);
f2[x_] = Log[(3 x + 6)/10];

plt = Plot[{f1[x], f2[x]}, {x, -3, 4},
   PlotRange -> {-5, 5}];

The intersections of the curves are

arg = Solve[{f1[x] == f2[x], -3/2 < x < 2}, x];

rng = LessEqual @@ Insert[x /. arg, x, 2];

f3[x_] = ConditionalExpression[f2[x], rng];

f4[x_] = ConditionalExpression[Min[f1[x], 0], rng];

Plot[{f1[x], f2[x], f3[x], f4[x]}, {x, -3, 4},
 Filling -> 3 -> {4},
 FillingStyle -> Lighter[ColorData[97][2], 0.7],
 PlotRange -> {-5, 5},
 PlotStyle -> {ColorData[97][1], ColorData[97][2],
   Opacity[0], Opacity[0]},
 PlotLegends -> Placed[
   {f1[x], f2[x], None, None},
   {0.8, 0.7}],
 AxesLabel -> (Style[#, 14] & /@ {x, f})]

Answer 3

As you did not specify any functions, therefore I choose something similar:

f1[x_] = -1 (2 x - 3) (x + 1);
f2[x_] = Log[(3 x + 6)/10];

To fill from f1 to the axis:

Plot[{f1[x], f2[x]}, {x, -3, 4}, PlotRange -> {-5, 5}, 
 Filling -> {2 -> 0}]

And to fill from f2 to f1:

Plot[{f1[x], f2[x], f3[x]}, {x, -3, 4}, PlotRange -> {-5, 5}, 
 Filling -> {2 -> {1}}]

Addendum

A fill from the axis down to f2 can be done with an auxillary function:

f3[x_] = If[Re@f2[x] < 0, 0, f2[x]]
Plot[{f3[x], f1[x], f2[x]}, {x, -3, 4}, PlotRange -> {-5, 5}, 
 Filling -> {1 -> {3}}]

For filling only inside f1, chnage the auxillary function and f2 because of the pole:

f2[x_] = Log[(3 x + 6.1)/10]; 
f3[x_] = If[f1[x] <= Re@f2[x], f2[x], If[Re@f2[x] < 0, 0, f2[x]]]
Plot[{f3[x], f1[x], f2[x]}, {x, -3, 4}, PlotRange -> {-5, 5}, 
 Filling -> {1 -> {3}}]