Question 375755
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)