Definition of the directional derivative:
▶ s → 0
+
−
▶ Rotate
s = 1.40
surface
z
=
f
(
x
,
y
)
vertical plane through
P
0
along
u
heights
f
(
P
0
),
f
(
P
) of the surface above
P
0
,
P
trace of the surface in the plane
secant through
(
P
0
,
f
(
P
0
)) and (
P
,
f
(
P
))
tangent at
(
P
0
,
f
(
P
0
)), slope (
D
u
f
)
P
₀
unit vector
u
displacement
s
u