1020.sagews

x, y = var('x y')
t, s = var('t s')
plot3d(x*y, (x, 0, 1.1), (y, 0, 1.1), aspect_ratio=1)+parametric_plot3d([cos(t), sin(t), 0], (t, 0, pi/2), color="black")

x, y = var('x y')
t, s = var('t s')
plot3d(x*y, (x, 0, 1.1), (y, 0, 1.1), aspect_ratio=1)+parametric_plot3d([cos(t), sin(t), cos(t)*sin(t)*s], (t, 0, pi/2), (s, 0, 1), color="red")

x, y = var('x y')
t, s = var('t s')
plot3d(x*y, (x, 0, 1.1), (y, 0, 1.1), aspect_ratio=1)+parametric_plot3d([cos(t), sin(t), cos(t)*sin(t)*s], (t, 0, pi/2), (s, 0, 1), color="red")+parametric_plot3d([cos(t), 0, cos(t)*sin(t)*s], (t, 0, pi/2), (s, 0, 1), color="green")