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Question: Using half angle formula, the exact value of cos 75° is:


A ) 2 -32
B ) - 2 +32
C ) - 2 -32
D ) 2 +32

Steps to derive

1 cos 75° = +1 + cos 150°2
 [Use cos x2 = +1 + cos x2
cos 75° is positive.
75° = 12 · 150°.]

2 = 1 + (-32)2 = 2 -34 = 2 -32
 [Use cos 150° = - 32.]



Hence the right answer is Option A

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