La funzione f(x) è la derivata della sua primitiva:
f
(
x
)
=
F
'
(
x
)
=
d
d
x
(
a
⋅
s
i
n
3
(
x
)
+
b
⋅
s
i
n
(
x
)
+
2
x
)
=
3
a
⋅
s
i
n
2
(
x
)
⋅
c
o
s
(
x
)
+
b
⋅
c
o
s
(
x
)
+
2
=
3
a
⋅
(
1
-
c
o
s
2
(
x
)
)
⋅
c
o
s
(
x
)
+
b
⋅
c
o
s
(
x
)
+
2
=
-
3
a
⋅
c
o
s
3
(
x
)
+
(
3
a
+
b
)
⋅
c
o
s
(
x
)
+
2
Da cui, eguagliando,:
-
3
a
⋅
c
o
s
3
(
x
)
+
(
3
a
+
b
)
⋅
c
o
s
(
x
)
+
2
=
c
o
s
3
(
x
)
-
3
⋅
c
o
s
(
x
)
+
2
→
a
=
-
1
3
;
b
=
-
2