Figures 6 - 9

> z := -4*x^3*y^2;

z := -4*x^3*y^2

> plot3d( z, x=-1..1, y=-1..1, title="Figure 6" );

[Maple Plot]

>

> z := x - 1/8*x^3 - 1/3*y^2;

z := x-1/8*x^3-1/3*y^2

> plot3d( z, x=-4..4, y=-4..4, title="Figure 7" );

[Maple Plot]

>

> z := x*y * exp( -x^2-y^2 );

z := x*y*exp(-x^2-y^2)

> plot3d( z, x=-3..3, y=-3..3, title="Figure 8" );

[Maple Plot]

>

> z := exp(-abs(x)) * cos( sqrt(x^2+y^2) );

z := exp(-abs(x))*cos(sqrt(x^2+y^2))

> plot3d( z, x=-5..5, y=-5..5, title="Figure 9" );

[Maple Plot]

>