Q. 16

# Mark the correct alternative in the following:

The value of cos (36^{o} – A) cos (36^{o} + A) + cos (54^{o} + A) cos (54^{o} – A) is

A. sin 2 A

B. cos 2A

C. cos 3A

D. sin 3A

Answer :

cos(54^{o} + A) =sin(36^{o} – A)

cos(54^{o} - A) =sin(36^{o} + A)

cos (36^{o}–A) cos (36^{o}+A)+ sin(36^{o}-A)sin(36^{o}+A)=cos(2A)

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