Add transparency to plots

eq = x ^ 2

x^2

Plot[eq, {x, -2, 2}]

-Graphics-

eq2 = -x ^ 2

-x^2

eq = x ^ 4 + y ^ 4 - 5 x ^ 2 * y ^ 2

x^4 - 5*x^2*y^2 + y^4

Plot3D[eq, {x, -3, 3}, {y, -3, 3}]

-Graphics3D-

eq2 = -100 - x ^ 4 - y ^ 4 + 5 x ^ 2 * y ^ 2

-100 - x^4 + 5*x^2*y^2 - y^4

Graphics[Disk[]]

-Graphics-

Row[Table[
	Graphics[{Opacity[n], Disk[]}, ImageSize -> Scaled[.1]],
	{n, 0.1, 1, 0.3}]]

-Graphics--Graphics--Graphics--Graphics-

Row[Table[
	Graphics[{Red, Opacity[n], Disk[]}, ImageSize -> Scaled[.1]],
	{n, 0.1, 1, 0.3}]]

-Graphics--Graphics--Graphics--Graphics-

Manipulate[RegionPlot[
	{x - y ≥ t, x + y ≥ s}, {x, -3 Pi, 3 Pi}, {y, -3 Pi, 3 Pi},
	PlotPoints -> 30,
	PlotStyle -> {Directive[Red], Directive[Blue, Opacity[0.5]]}
	],
	Style["Solving the system of inequalities: x-y ≥ t and x +y ≥ s", Bold, 12],
	{{s, 0}, -20, 20}, {{t, 0}, -20, 20}
	]

Manipulate[RegionPlot[{x - y >= t, x + y >= s}, {x, -3*Pi, 3*Pi}, {y, -3*Pi, 3*Pi}, PlotPoints -> 30, PlotStyle -> {Directive[Red], Directive[Blue, Opacity[0.5]]}], Solving the system of inequalities: x-y ≥ t and x +y ≥ s, {{s, 0}, -20, 20}, {{t, 0}, -20, 20}]