Section 14.1 (6th edition)

load(draw);

Example 6: 3d graph: view=[xangle, zangle] changes the view rotated by xangle degrees (0 to 360) around x-axis and by zangle (0 to 360) around z-axis. 

draw3d(view=[30,135],
       explicit(sqrt(9-x^2-y^2),x,-3,3,y,-3,3),
       xlabel="x", ylabel="y", surface_hide=true,
       user_preamble="set size 1,1.25");

3d graph: enhanced3d=true fills the entire surface with a color gradient, which depends on the z-axis. 

draw3d(view=[30,135], enhanced3d=true,
       explicit(sqrt(9-x^2-y^2),x,-3,3,y,-3,3),
       xlabel="x", ylabel="y", surface_hide=true,
       user_preamble="set size 1,1.25");

Figure 20 Part I: Graph

draw3d(enhanced3d=true, view=[30,135],
       explicit(-x*y*exp(-x^2-y^2),x,-2,2,y,-2,2),
       xlabel="x", ylabel="y");

Level Curves: contour_levels=[zmin,increment,zmax] determines the level curves of heights from zmin to zmax by value of increment, but some lines may not appear in the graph

wxdraw3d(color=red, contour=map, contour_levels=[-0.2,0.02,0.2],
       explicit(-x*y*exp(-x^2-y^2),x,-2,2,y,-2,2),
       user_preamble="unset clabel; set size 0.8,1.2", xlabel="x", ylabel="y");

Figure 20 Part II: Graph

draw3d(enhanced3d=true,  view=[45,210],
       explicit(-3*y/(1+x^2+y^2),x,-5,5,y,-5,5),
       xlabel="x", ylabel="y");

Level Curves: You will find the corresponding level curves.

wxdraw3d(color=red, contour=map, contour_levels=[-1.5,0.05,1.5],
       explicit(-3*y/(1+x^2+y^2),x,-5,5,y,-5,5),
       user_preamble="unset clabel");

*Exercise 30(c): Draw the graph of z = 1/(1+x^2+y^2)

wxdraw3d(enhanced3d=true,
       explicit(1/(1+x^2+y^2),x,-3,3,y,-3,3),
       user_preamble="unset colorbox");

*Exercise 56: Draw the graph and the contour map for z = exp(x)*cos(y)

wxdraw3d(enhanced3d=true,
       explicit(exp(x)*cos(y),x,-0.2,2,y,-2*%pi,2*%pi),
       user_preamble="unset colorbox");

wxdraw3d(color=red, contour=map, contour_levels=15,
       explicit(exp(x)*cos(y),x,-0.2,2,y,-3*%pi,3*%pi),
       user_preamble="unset clabel");

*Exercise 60: Draw the graph and the contour map for z = (x-y)/(1+x^2+y^2)

wxdraw3d(enhanced3d=true,
       explicit((x-y)/(1+x^2+y^2),x,-5,5,y,-5,5),
       user_preamble="unset colorbox");

wxdraw3d(color=red, contour=map, contour_levels=15,
       explicit((x-y)/(1+x^2+y^2),x,-5,5,y,-5,5),
       user_preamble="unset clabel");