SOLUTION: Find the exact value using a sum or difference identity. cos(165°)

Algebra ->  Trigonometry-basics -> SOLUTION: Find the exact value using a sum or difference identity. cos(165°)       Log On


   

Question 849128: Find the exact value using a sum or difference identity. cos(165°)
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
Find the exact value using a sum or difference identity. cos(165°)
----------
cos(165) = -cos(15) = -cos(60-45) = -[cos(60)cos(45)+sin(60)sin(45)]
---------
= -[(1/2)(sqrt(2)/2) + (sqrt(3)/2)(sqrt(2)/2)]
---------
= -[(sqrt(2)/4) + sqrt(6)/4]
------
= -[sqrt(2)+sqrt(6)]/4
==========================
Cheers,
Stan H.
==========================