SOLUTION: Use a sum or difference formula to find the exact value of the following. cos 2(pi)/5 cos(pi)/15 + sin2(pi)/5 sin(pi)/15

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Question 1060407: Use a sum or difference formula to find the exact value of the following.
cos 2(pi)/5 cos(pi)/15 + sin2(pi)/5 sin(pi)/15
Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
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cos%282pi%2F5%29%2Acos%28pi%2F15%29+%2B+sin%282pi%2F5%29%2Asin%28pi%2F15%29 = cos%282pi%2F5-pi%2F15%29 = cos%286pi%2F15-pi%2F15%29 = cos%285pi%2F15%29 = cos%28pi%2F3%29 = 1%2F2.


The sum or difference formulas for cosine are

cos(a+b) = cos(a)*cos(b) - sin(a)*sin(b),

cos(a-b) = cos(a)*cos(b) + sin(a)*sin(b).