1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Solve this lim

  1. Nov 6, 2009 #1
    lim
    x->pi/2 (1+cos2x)/cos3x

    i tried all the identities and got

    2cos^2(x)/cos(3x)

    what now? i used cos(a+b)=cos(a)*cos(b)-sin(a)sin(b)
    but no help there
     
  2. jcsd
  3. Nov 6, 2009 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    I don't think you tried 'all of the identities'. The denominator hasn't changed at all. What can you do to it?
     
  4. Nov 7, 2009 #3
    the best i could come up with is

    lim
    x->pi/2 (1+cos2x)/cos3x

    lim
    x->0 2cos^2(pi/2-x)/cos(3(pi/2-2))

    lim
    x->0 (2sin^2(x))/sin(3x)

    lim
    x->0 2sin(x)*-1/3

    =0
     
    Last edited: Nov 7, 2009
  5. Nov 7, 2009 #4

    lanedance

    User Avatar
    Homework Helper

    can you use L'hopitals? good to check with anyway...
     
  6. Nov 7, 2009 #5

    lanedance

    User Avatar
    Homework Helper

    EDIT: as below

    in the mean time its worth trying Dick's suggestion though...
     
    Last edited: Nov 7, 2009
  7. Nov 7, 2009 #6

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Ok, you can do it that way. cos(3(pi/2-x))=(-sin(3x)), though.
     
  8. Nov 7, 2009 #7

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    I guess I was assuming the OP knew lim sin(x)/sin(3x)=(1/3) and didn't think sin(x)/sin(3x)=(1/3). Hope I wasn't wrong.
     
  9. Nov 7, 2009 #8

    lanedance

    User Avatar
    Homework Helper

    nope, my bad, made a quick arithmetic error, all looks good
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Solve this lim
  1. Lim of (Replies: 4)

  2. Lim sup An lim inf An (Replies: 2)

Loading...