Plot[(x^2),{x,-2,2},AspectRatio->Automatic,GridLines->Automatic]

Options[Plot]

N[(1/GoldenRatio)]

Sqrt[2]

Sum[(1/(n^2)),{n,1,∞}]

Plot3D[{0,(x^2)+(y^2)},{x,-3/2,3/2},{y,-3/2,3/2},Mesh->False,PlotStyle->{Opacity[0.3],LightGreen},RegionFunction->Function[{x,y,z},x^2+y^2≤9/4],BoundaryStyle->{Thick,Blue}]

ContourPlot[(x^2)+(y^2),{x,-3/2,3/2},{y,-3/2,3/2}]

ContourPlot[(x^2)+(y^2),{x,-3/2,3/2},{y,-3/2,3/2},ContourShading->False,ContourLabels->True]

ContourPlot[(x^2)+(y^2),{x,-3/2,3/2},{y,-3/2,3/2},ContourShading->False,ContourLabels->True,Contours->{0.16,0.25,0.5,1,1.5,2,2.5,3,3.5,4}]

ContourPlot[(x^2)+(y^2)==1,{x,-3/2,3/2},{y,-3/2,3/2}]

Plot3D[{0,(x^2)+(y^2)},{x,-3/2,3/2},{y,-3/2,3/2},Mesh->False,PlotStyle->{Opacity[0.3],LightGreen},RegionFunction->Function[{x,y,z},x^2+y^2≤9/4],BoundaryStyle->{Thick,Blue},BoxRatios->Automatic]

ContourPlot3D[(x^2)+(y^2)-z==0,{x,-3/2,3/2},{y,-3/2,3/2},{z,0,3},Mesh->False,RegionFunction->Function[{x,y,z},x^2+y^2≤9/4],BoxRatios->Automatic]