Posted by Anonymous on .
1.Write the expression as a sum or difference.
cos(5x)cos(4x)
2.Write the expression as a product.
cos(4x)cos(12x)

Precalculus 
MathMate,
1. use
cos(a)+cos(b)=2[cos((a+b)/2)cos((ab)/2)]
and set a=x/2, b=9x/2
2. use
cos(a)cos(b)=2[sin((a+b)/2)sin((ab)/2)]
and set a=4x, b=8x
Hint: before doing the work, check the formulas using some trivial cases such as a=π/2, and b=π/4.