SOLUTION: cos(120) +cos(30) Using the product to sum formula

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Question 945993: cos(120) +cos(30)
Using the product to sum formula
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

cos%28120%29+%2Bcos%2830%29+
you are supposed to express cos%28120%29 as cos%2890+%2B+30%29 and change it to single angles with the standard formula
cos%28a+%2B+b%29+=+cos%28a%29%2Acos%28b%29+-+sin%28a%29%2Asin%28b%29
cos%2890%2B30%29+%2Bcos%2830%29
=cos%2890%29%2Acos%2830%29+-+sin%2890%29%2Asin%2830%29%2Bcos%2830%29 ........since cos%2890%29=0, sin%2890%29=1, we have
=0%2Acos%2830%29+%2Bcos%2830%29+-+1%2Asin%2830%29

=cos%2830%29+-+sin%2830%29

=sqrt%283%29%2F2-1%2F2