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Precalculus

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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 -

    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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