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Homework Help: Trig Identity

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data
    cos^2x - cos^4x = cos^2x sin^2x

    2. Relevant equations

    3. The attempt at a solution

    L.S. = cos^2x -(cos^2x)(cos^2x)
    = cos^2x -cos^2x(cos^2x)

    I'm stuck here. Am I doing something wrong during the first step? Thanks for your help.
  2. jcsd
  3. Jan 8, 2008 #2
    What are you trying to solve for, x? Or are you trying to simplify it?
  4. Jan 8, 2008 #3
    It's trig identity. Trying to make the left side look like the right side.
  5. Jan 8, 2008 #4
    manipulate the right side, it's easier ... you want it all in terms of cosine
  6. Jan 8, 2008 #5


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    Science Advisor

    Didn't we just do one like this? cos^2(x)- cos^4(x)= cos^2(x)- cos^2(x) cos^(x)= cos^2(x)(1- cos^2(x)). Does that help?
    Last edited by a moderator: Jan 8, 2008
  7. Jan 8, 2008 #6
    Sort of but it's too cluttered.

    If I try to manipulate the right side, I get this:

    = (cos^2x)(sin^2x)
    = (cos^2x)(1-cos^2x)

    After that, I don't know how to make the cos^4 on the left side.
  8. Jan 8, 2008 #7
    foil ...
  9. Jan 8, 2008 #8
    God... how did I miss that. Thanks a lot.

    I'm just really tense. Got a huge test tomorrow and trying to cram it all. A week for stuff in a day isn't a good idea... Thanks again!
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