SOLUTION: Use an identity to find the exact value of the expression: Note: You are not allowed to use decimals in your answer. cos(202.5)

Algebra ->  Trigonometry-basics -> SOLUTION: Use an identity to find the exact value of the expression: Note: You are not allowed to use decimals in your answer. cos(202.5)       Log On


   

Question 952158: Use an identity to find the exact value of the expression:
Note: You are not allowed to use decimals in your answer.
cos(202.5)
Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
cos(202.5) = cos(405/2)
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The half-angle formula for cosine is tedious. Cos^2 is much simpler.
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cos^2(405/2) = (1 + cos(405))/2
= %281+%2B+sqrt%282%29%2F2%29%2F2+=+%282+%2B+sqrt%282%29%29%2F4
cos%28202.5%29+=+-sqrt%282+%2B+sqrt%282%29%29%2F2 (negative in Q3)