Published by:
CGP EDU Academic Team
Published on: August 13, 2026
If 5(tan 2 x – cos 2 x) = 2cos2x + 9, then the value of cos4x is :
A
B
C
D
Text Solution
Verified by ExpertsThe correct answer is:
D
Sol. 5(tan 2 x – cos 2 x) = 2cos2x + 9
5 
= 
5(tan 4 x + tan 2 x – 1) = 2 – 2 tan 2 x + 9 + 9tan 2 x
5tan 4 x – 2tan 2 x – 16 = 0
5tan 4 x – 10tan 2 x + 8tan 2 x – 16 = 0
5tan 2 x (tan 2 x – 2) + 8 (tan 2 x – 2) = 0
(5tan 2 x + 8) (tan 2 x – 2) = 0
tan 2 x= 2
cos2x = 
= 
cos4x = 2cos 2 2x – 1 = 
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