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Help with tangent math question

  1. Jul 24, 2010 #1
    Hello Everyone,

    I'm doing my math in advance so I came across a Trigonometry question I came across in my textbook. I did make some progress but I do not know how to go about it further.

    1. The problem statement, all variables and given/known data

    Prove that,

    tan3A + tan2A + tanA = tan3Atan2AtanA

    3. The attempt at a solution

    I did simplify tan3A as- tan(2A+A) and then tried going about it. I just need someone to spur me on to the right approach and not provide the answer necessarily.
  2. jcsd
  3. Jul 24, 2010 #2


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    Homework Helper

    But I can't...

    ...since it's generally not true!

    The expansion of tan(A+B) is given by [tex]tan(A+B)=\frac{tanA+tanB}{1-tanAtanB}[/tex] but if you know the expansion of sin(A+B) and cos(A+B) then [tex]tan(A+B)=\frac{sin(A+B)}{cos(A+B)}[/tex]
  4. Jul 24, 2010 #3


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    As Mentallic says, you cannot "prove" this because it is not true. For a counter example, take practically any values for A, say "A= 1". Is it possible that the problem was not to "prove an identity" but to solve for a value of A that makes the equation true?
  5. Jul 24, 2010 #4
    Thank You! Must have been a typing error in the book. Yes, I do know the expansion of tan(A + B) Thnks anyways. I'll re-check the question and get back to you.
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