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Trig Identities:

  1. Jan 26, 2009 #1
    Everyone knows the obvious trig identities like sin^2 + cos^2 =1, cosx=1+ sin^2, and tanx =sin/cos. I ran across an old identity the other day: 3cos^2(3x)+3sin^2(3x)=3. Can anyone here figure out why and how? I tried it and couldnt figure it out.
  2. jcsd
  3. Jan 26, 2009 #2


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    Look at your first "obvious trig identity"...
  4. Jan 26, 2009 #3
    3cos^2(3x)+3sin^2(3x)=3[cos^2(3x)+sin^2(3x)] , now substitude 3x=z and using sin^2(z) + cos^2(z) =1 you have it!

    NB: I´m not sure this is correct, whatever it is meant to be: cosx=1+ sin^2
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