SOLUTION: how do u find exact radical value of cos 165?

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Question 308216: how do u find exact radical value of cos 165?
Answer by nyc_function(2741) About Me  (Show Source):
You can put this solution on YOUR website!
Use the cosine angle addition formula.
cos(165°)
= cos(120° + 45°)
= cos(120°)cos(45°) - sin(120°)sin(45°)
= (-1/2)(√2/2) - (√3/2)(√2/2)
= -√2/4 - (√3√2)/4
= -√2/4 - √6/4
= (-√2 - √6)/4
Therefore, cos(165°) = (-√2 - √6)/4.
Hope this helps!