Published by:
CGP EDU Academic Team
Published on: August 13, 2026
Prove that 4(cos 3 20 ° + cos 3 40 ° ) = 3(cos 20 ° + cos 40 ° )
Text Solution
Verified by ExpertsThe correct answer is:
CHECK THE SOLUTION.
LHS = 4 (cos 20 ° + cos 40 ° ) (cos 2 20 ° + cos 2 40 ° – cos 20 ° cos 40 ° )
= 4 (cos 20 ° + cos 40 ° ) 
= 
(cos 20 ° + cos 40 ° ) (2 + cos 80 ° + cos 40 ° – 
– cos 20 ° )
= 2 (cos 20 ° + cos 40 ° ) (2 + 2 cos 60° cos20° – 
– cos 20 ° )
= 2 (cos 20 ° + cos 40 ° ) (2 + cos 20 ° – 
– cos 20 ° )
= 2 × 
(cos 20 ° + cos 40 ° ) = 3 (cos 20 ° + cos 40 ° ) = RHS
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