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Homework Help: Relearning a little bit of trig

  1. Nov 24, 2008 #1
    I'm trying to relearn some trig for a math placement test to put me into a calculus class and am having trouble with one particular problem. One of the example problems is as follows:

    Find cos^2(2x)+sin^2(2x)

    I know the trig identity cos^2(x)+sin^2(x)=1 but I don't know how to apply this, if it should be applied at all, to the problem given. Could I get some help?
  2. jcsd
  3. Nov 24, 2008 #2


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    Let u = 2x. Then...
  4. Nov 27, 2008 #3
    then where?
  5. Nov 27, 2008 #4
    you need to see what do they wan to acquire when you simplify it.. you gotta see the factors in the RHS or while you are solving.
  6. Nov 27, 2008 #5
    yeah, well cos^2(u)+sin^2(u)=1
    therefore cos^2(2x)+sin^2(2x)=1

    ie as long as the two angles are the same, the sum of the sin squared and cos squared of that angle is always 1.
  7. Nov 27, 2008 #6
    he meant when to apply tat substitution whr by we sub in cos²(2x)+sin²(2x) = 1
    or when to leave it alone.
  8. Nov 27, 2008 #7
    ok, I thought it was a simpler question than that, thought he didn't quite grasp the identity concept, that's all. We very seldom sub in cos²(2x)+sin²(2x) = 1 but rather cos²(x)+sin²(x) = 1.
  9. Nov 27, 2008 #8
    exactly! for the thread starter: practice is the key ! :smile:
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