Trigonometry
Use a product-to-sum formula to write the given trigonometric expression as a sum or difference:
9 cos 5x cos 8x
(9/2)[cos (13x) - cos (3x)]
(9/2)[cos (13x) + cos (3x)]
(9/2)[sin (13x) + cos (3x)]
(9/2)[cos (13x) + sin (3x)]