1
You visited us
1
times! Enjoying our articles?
Unlock Full Access!
Byju's Answer
Standard XII
Mathematics
Principal Solution of Trigonometric Equation
Find : cos2...
Question
Find :
cos
2
π
7
+
cos
4
π
7
+
cos
6
π
7
+
3
2
Open in App
Solution
cos
2
π
7
+
cos
4
π
7
+
cos
6
π
7
+
1
2
to multiply by
sin
π
7
sin
π
7
cos
2
π
7
+
sin
π
7
cos
4
π
7
+
sin
π
7
cos
6
π
7
+
1
2
sin
π
7
⇒
sin
A
cos
B
=
1
2
[
sin
(
A
−
B
)
+
sin
(
A
+
B
)
]
⇒
1
2
[
sin
(
π
2
−
2
π
7
)
+
sin
(
π
7
+
2
π
7
)
]
+
1
2
[
sin
(
π
7
−
4
π
7
)
+
sin
(
π
7
+
4
π
7
)
]
+
1
2
[
sin
(
π
7
−
6
π
7
)
+
sin
(
π
7
+
6
π
7
)
]
+
1
2
sin
π
7
multiply by 2:
⇒
−
sin
π
7
+
sin
3
π
7
−
sin
3
π
7
+
sin
5
π
7
−
sin
5
π
7
+
sin
7
π
7
+
sin
π
7
−
sin
π
7
+
sin
π
+
sin
π
7
=
0
3
2
−
1
=
1
2
∴
cos
2
π
7
+
cos
4
π
7
+
cos
6
π
7
+
1
2
=
0
⇒
cos
2
π
7
+
cos
4
π
7
+
cos
6
π
7
+
1
2
+
1
=
0
+
1
=
1
.
Suggest Corrections
0
Similar questions
Q.
Find
cos
2
π
7
+
cos
4
π
7
+
cos
6
π
7
Q.
c
o
s
2
π
7
+
c
o
s
4
π
7
+
c
o
s
6
π
7
=