SOLUTION: Use a sum-to-product formula to rewrite cos 5a + cos 4a as a product

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Question 375755: Use a sum-to-product formula to rewrite
cos 5a + cos 4a
as a product
Answer by vleith(2983) About Me  (Show Source):
You can put this solution on YOUR website!
See this URL for various sum-to-product formula
http://library.thinkquest.org/17119/lg/memorize/s_p.html
cos 5a + cos 4a = 2 cos((5a+4a)/2) cos((5a-4a)/2)
cos 5a + cos 4a = 2 cos((9a)/2) cos(a/2)