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Confused about double angle identities

  1. Dec 4, 2007 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    sin2x = 2sinxcosx

    3. The attempt at a solution

    LS = sin(2x + 2x) - sin2x/sin2x
    = sin2xcos2x + cos2xsin2x - sin2x/sin2x
    = 2sin2xcos2x - sin2x/sin2x
    This is where i get stuck...
    I don't know what happens if you try to: 2sinx2x - sin2x or 2sin2x/sin2x, is that possible or not? Can you guys help me solve this question? Thanks.
  2. jcsd
  3. Dec 4, 2007 #2


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    Homework Helper

    Quick question you are to prove that

  4. Dec 4, 2007 #3
  5. Dec 4, 2007 #4


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    Gold Member

    I believe there are two more Relevant equations you need.
  6. Dec 4, 2007 #5
    dosen't matter i was able to solve it, but here's another problem which I'm now stuck with. [tex]cosx+cos2x+cos3x=cos2x(1+2cosx)[/tex].
    I've tried many things with this problem, but always seem to get lost.
    The only given identites which I'm given to use:
    Last edited: Dec 4, 2007
  7. Dec 4, 2007 #6


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    OK, since you have to SOLVE and not prove....
    if you expand the RHS you would see that the cos2x cancels out and you are left with


    then expand out cos(3x) and see if anything gets simpler
  8. Dec 4, 2007 #7
    lol, sorry for not stating this, but i have to prove not solve.
  9. Dec 4, 2007 #8


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    well then expand out the LHS
    Recall that cos(3x)=cos(2x+x)
  10. Dec 4, 2007 #9
    yea then i get: cos2xcosx + sin2xsinx + cos2x + cosx
  11. Dec 4, 2007 #10


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    and sin2x=2sinxcosx

    expand out and find all in terms of cosx and hopefully it will work
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