Posted by **Anonymous** on Monday, December 6, 2010 at 10:17am.

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**, Monday, December 6, 2010 at 10:34am
1. use

cos(a)+cos(b)=2[cos((a+b)/2)cos((a-b)/2)]

and set a=x/2, b=9x/2

2. use

cos(a)-cos(b)=-2[sin((a+b)/2)sin((a-b)/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.

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