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Limits of trig functions

  1. Nov 22, 2011 #1
    1. The problem statement, all variables and given/known data
    lim(x -->0) (1-cos(14x))/(xsin(18x))

    2. Relevant equations
    None?


    3. The attempt at a solution
    The hint tells me to use L'hopital's rule through which I got

    lim(x-->0) (sin(14x))/(18xcos(18x)+sin(18x)) (I factored out the 14 in the numerator)

    That gave me a 0/0 so I did L'hopital's rule again, through which I got

    lim(x-->0) (cos(14x))/(cos(18x)-9xsin(18x)) (I factored out 14/2)

    This gives me 98, which is wrong. (As x-->0, the function becomes 1, leaving what I factored out as 98)

    I'm not sure what to do here. Help? :(
     
    Last edited: Nov 22, 2011
  2. jcsd
  3. Nov 22, 2011 #2

    Dick

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    I think it's the things you are factoring out. I don't think you factor out 14/2 to get the expression you showed at the end. Can you show how you did that? Try showing your expressions without the 'factoring out'.
     
    Last edited: Nov 22, 2011
  4. Nov 22, 2011 #3
    lim(x-->0) (1-cos14x)/(xsin(18x)), this gives 0/0

    L'hopital's

    lim(x-->0) (14sin14x)/(sin(18x) + 18x(cos(18x))), Gives 0/0

    L'hopital's

    lim(x-->0) (196cos(14x))/(36cos(18x)-324xsin(18x)) This gives 49/9 which is right...

    Thanks, it seems I just screwed up the calculations. Sorry for the bother
     
  5. Nov 22, 2011 #4

    Dick

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    You are welcome. No problem.
     
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