Published by:
CGP EDU Academic Team
Published on: August 13, 2026
Prove that (4cos 2 9 º – 3) (4cos 2 27 º – 3) = tan 9 º
Text Solution
Verified by ExpertsThe correct answer is:
CHECK THE SOLUTION.
We have cos3x = 4 cos 3 x – 3 cosx, so 4 cos 2 x – 3 = 
for all x ≠ (2k + 1). 
, k ∈ Z. Thus
(4cos 2 9 º – 3) (4cos 2 27 º – 3) = 
. 
= 
= tan 9 º
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