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Easy trig stuffI jsut feeling dumb

  1. Oct 29, 2006 #1
    ok.. title says it all I feeling really stupid right now and seem to be having trouble with elementary stuff here..
    given r= sin 3x

    dr= cos 3x * 3

    i got that . but next step in the equation i am doing is

    3cos 3x * cos x

    as i said feeling stupid si it

    3 cos^2 3x?
  2. jcsd
  3. Oct 29, 2006 #2


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    You can only combine trig functions like that when the arguments are equal.
  4. Oct 29, 2006 #3
    It's certainly not. If you're in doubt, write it as 3(cos(3x))^2.

    To rewrite cos 3x * cos x, employ the identities involving cos(A+B) and cos(A-B).
  5. Oct 29, 2006 #4
    ok i understna what you wrote neutrino
    so i but i got that 3 out front so with the little identity bit wuld it be

  6. Oct 29, 2006 #5
    Yes, if it makes the problem any simpler.
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