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Math 12 - trig question?

  1. Jan 21, 2009 #1
    1. Determine an equivalent expression for sin 3x cos x + cos 3x sin x .
    A. 4sin x
    B. 2sin x cos x
    C. 4sin x cos x
    D. 2sin 2x cos 2x

    2. sin(a+b) = sinacosb+cosasinb, sin2x = 2sinxcosx



    3. This question appeared on a practice final I was attempting at. It's on a no calculator section so I'm having difficulty solving it without one. I got up to

    sin 3x cos x + cos 3x sin x
    = sin(3x + x)
    = sin(4x)

    The answer is 2sin 2x cos 2x, I know that sin2x = 2sinxcosx but I don't know how to expand it when sin2x becomes sin4x. It'd be great if someone could explain it. Thanks!
     
    Last edited: Jan 21, 2009
  2. jcsd
  3. Jan 21, 2009 #2

    rock.freak667

    User Avatar
    Homework Helper

    sin2A=2sinAcosA

    if A=2x

    sin4x=2sin2xcos2x
     
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