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The general s...
Question
The general solution of
cos
3
x
+
8
cos
3
x
=
0
is
.
Open in App
Solution
Given:
cos
3
x
+
8
cos
3
x
=
0
To find the general solution, follow the steps:-
Transform the equation into product of linear factors
Equate each factor to zero
Solve each equation individually
Take union of all these solutions
To make the arguments of cosine uniform, we apply the formula
cos
3
x
=
4
cos
3
x
−
3
cos
x
The resulting equation is
4
cos
3
x
−
3
cos
x
+
8
cos
3
x
=
0
⇒
12
cos
3
x
−
3
cos
x
=
0
(Dividing by
3
on both side)
⇒
4
cos
3
x
−
cos
x
=
0
⇒
cos
x
(
4
cos
2
x
−
1
)
=
0
⇒
(
cos
x
−
0
)
(
4
cos
2
x
−
1
)
=
0
Here, either
cos
x
=
0
or
4
cos
2
x
−
1
=
0.
For
cos
x
=
0
,
general solution is
x
=
(
2
n
+
1
)
π
2
,
n
∈
Z
.
Represent the above solution set as
A
.
For
4
cos
2
x
−
1
=
0
,
cos
2
x
=
1
4
⇒
cos
2
x
=
(
1
2
)
2
⇒
cos
2
x
=
(
cos
π
3
)
2
The general solutoin, say B, is
x
=
n
π
±
π
3
,
n
∈
Z
.
The complete general solution is
A
∪
B
,
i.e.,
x
∈
{
(
2
n
+
1
)
π
2
}
⋃
{
n
π
±
π
3
}
,
n
∈
Z
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