Sphere
f
(
x
1
,
x
2
)
=
x
1
2
+
x
2
2
f(x_1, x_2) = x_1^2 + x_2^2
f
(
x
1
,
x
2
)
=
x
1
2
+
x
2
2
Contour plot
Superficie 3D