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Homework Help: Trigonometric Expression

  1. Dec 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Express the function f(x) = -2sin(3x) -4cos(3x) in the form of a general sine finction.

    Identify the amplitude, period, and phase shift

    2. Relevant equations

    sinx/cosx = tanx

    sin2x + cos2x = 1

    3. The attempt at a solution

    don't know how to start
    don't penalize me for this.
  2. jcsd
  3. Dec 5, 2012 #2


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    If you wanted to express cos(x) as sin(x+a), then a is the phase shift.
    What would a have to be?
  4. Dec 6, 2012 #3


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    A sin(B x + C) + D=A sin(C) cos(B x)+A cos(C) sin(B x)+D
    so solve simultaneously the equations

    A cos(C) sin(B x)=-2sin(3x)
    A sin(C) cos(B x)=-4cos(3x)
  5. Dec 6, 2012 #4


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    The equations are valid for every value of x. The zeroes of sin(Bx) are at x=k∏/3, k integer. What should be B?

    What are the equations at x=0, x=π/6?

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