math(advanced algebra&trigonometry)

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the expression cos4x cos3x + sin4x sin3x is equivalent to
which one of the following is the answer:
1) sinx
2) sin7x
3) cosx
4) cos7x

  • math(advanced algebra&trigonometry) -

    Look up the equation for cos(x-y) and it should be obvious what the answer is.

    Think of x as 4x and y as 3x.

    http://www.csgnetwork.com/trigtriformulatables.html

  • math(advanced algebra&trigonometry) -

    it is cosx
    thanks

  • math(advanced algebra&trigonometry) -

    Seca=4x + 1/x than secb + tana what please tell me after 2days

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