For each of the following two expressions, apply either the product-to-sum or sum-to-product formula, as appropriate.
(1) sin(7x) sin(2x)
(2) cos(8x) + cos(4x)
a) sin(7x) cos(5x) - cos(7x) sin(5x)
b) cos(6x) cos(2x) + sin(6x) sin(2x)
c) sin(5x) + sin(9x)
d) sin(6x) sin(2x)