F
a
(
x
+
Δx
)
−
F
a
(
x
)
=
f
(
ξ
)
·
Δx
⇒
f
(
ξ
)
=
F
a
(
x
+
Δx
)
−
F
a
(
x
)
Δx
f
(
x
)
=
lim
Δx
→
0
F
a
(
x
+
Δx
)
−
F
a
(
x
)
Δx
=
F
a
'
(
x
)