Example 1

f1=Plot3D[Sqrt[x*y],{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1}];Show[f1]

t1=Plot3D[1+(x-1)/2+(y-1)/2,{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1},PlotStyle->Green];Show[f1,t1]

Example 2

f2=Plot3D[(x*y)/(x^2+y^2),{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1}];Show[f2]

t2=Plot3D[0,{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1},PlotStyle->Green];Show[f2,t2]

px=Plot3D[(y^3-x^2*y)/(x^2+y^2)^2,{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1}];Show[px]

py=Plot3D[(x^3-y^2*x)/(x^2+y^2)^2,{x,-2,2},{y,-2,2},AxesLabel->{x,y,z},ViewPoint->{1,1,1}];Show[py]