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

Question 308216: how do u find exact radical value of cos 165?
Answer by nyc_function(2741) (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!

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