f
(
x
)
l
n
(
x
)
g
(
x
)
c
o
s
(
x
)
D
x
(
f
(
x
)
g
(
x
)
)
=
g
(
x
)
′
f
(
x
)
+
f
(
x
)
′
g
(
x
)
=
c
o
s
(
x
)
1
x
+
l
o
g
(
x
)
-
s
i
n
(
x
)
=
c
o
s
(
x
)
x
-
l
o
g
(
x
)
s
i
n
(
x
)