f
(
x
,
y
)
=
-
2
x
-
2
y
+
4
∇
f
(
0
,
0
)
=
(
0
,
0
)
|
|
∇
f
(
0
,
0
)
|
|
=
0
D
u
f
(
0
,
0
)
=
0
s
n
a
p
t
o
g
r
i
d
s
h
o
w
g
r
a
d
i
e
n
t
∇
f
s
h
o
w
u
n
i
t
d
i
r
e
c
t
i
o
n
v
e
c
t
o
r
u