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Mathematics
Mathematics
Problem
If
5
(
tan
2
x
−
cos
2
x
)
=
2
cos
2
x
+
9
5(\tan^2 x - \cos^2 x) = 2\cos 2x + 9
5
(
tan
2
x
−
cos
2
x
)
=
2
cos
2
x
+
9
, then the value of
cos
4
x
\cos 4x
cos
4
x
is : [2017] (a)
−
7
9
-\frac{7}{9}
−
9
7
(b)
−
3
5
-\frac{3}{5}
−
5
3
(c)
1
3
\frac{1}{3}
3
1
(d)
2
9
\frac{2}{9}
9
2
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