1827.sagews

var('x,y')
f=sin(x^2+y^2+x*y)
plot3d(f, (x,-2,2), (y,-2,2)) + text3d("sin(x^2+y^2+x*y)",(1,1,2))

var('x,y')
plot3d(x*y/(x^2+y^2), (x,-2,2), (y,-2,2)) + text3d("x*y/(x^2+y^2)",(1,1,1))

var('x,y')
plot3d(x^2*y/(x^2+y^2), (x,-2,2), (y,-2,2)) + text3d("x^2*y/(x^2+y^2)",(1,1,2))

var('x,y')
plot3d(x^2*y/(x^4+y^2), (x,-2,2), (y,-2,2)) + text3d("x^2*y/(x^4+y^2)",(1,1,1.2))