Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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
    dx

    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

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No.

    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

    3(1/2(cos(a-b)+cos(a+b)))
     
  6. Oct 29, 2006 #5
    Yes, if it makes the problem any simpler.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook