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: L'Hopital Rule

  1. Sep 28, 2010 #1
    1. The problem statement, all variables and given/known data
    [tex] \lim_{x\to0} \frac{\arcsin(x)-x}{x^3} [/tex]

    3. The attempt at a solution
    This is obviously an indeterminate form, so we apply L'hopital's rule to get

    [tex] \lim_{x\to0} \frac{\frac1{\sqrt{1-x^2}} - 1 }{ 3x^3} [/tex]
    which is again an indeterminate form so we apply it again to get
    [tex] \lim_{x\to0} \frac{(1-x^2)^{-\frac32}}6 [/itex]
    from which the solution is obviously [itex] \frac16 [/itex].

    However, this is my question. After the first application of L'Hopital, we could have simplified
    [tex] \lim_{x\to0} \frac{\frac1{\sqrt{1-x^2}} - 1 }{ 3x^3} = \lim_{x\to0} \frac{1-\sqrt{1-x^2}}{3x^2\sqrt{1-x^2}} [/tex]
    This is no longer an indeterminate form and would suggest that the limit does not exist. Is there any justification for why this can't be done? Possibly, do we know that either: 1) This simplification is not permitted after applying L'Hopital or 2) We know the limit exists and is finite and so are forced to apply L'Hopital yet again?
  2. jcsd
  3. Sep 28, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Why do you think lim x->0 of (1-sqrt(1-x^2))/(3*x^2*sqrt(1-x^2)) isn't indeterminant? It looks like 0/0 to me.
  4. Sep 28, 2010 #3
    Haha, yes. The ability to subtract has apparently escaped me. Thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook