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Proving this trig identity

  1. May 2, 2005 #1
    This is a tricky one, sin(3x)cos(x)=sin(x)cos(x)(3-sin^2(x))

    I have tried addition formulas and expanding sin(3x) into something else, any help?
  2. jcsd
  3. May 2, 2005 #2


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    That's not just "tricky", it's false! Try [tex]x= \frac{\pi}{4}[/tex].
  4. May 2, 2005 #3


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    Like Halls said, remember that an identity is an equation that all values are solutions.
  5. May 3, 2005 #4


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    It should have been

    [tex]\cos x\sin 3x\equiv \cos x\sin x\left(3-4\sin^{2}x\right) [/tex]

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