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Homework Help: L'Hopital's rule problem

  1. May 5, 2010 #1
    1. The problem statement, all variables and given/known data
    lim (sin4x) divided by (x^2+8x)
    x approaches 0

    2. Relevant equations
    L'Hopital's rule

    3. The attempt at a solution

    u = sin4x
    du = cos4x

    y = x^2+8x
    dy = 2x+c
  2. jcsd
  3. May 5, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    The derivative would be called du/dx, not just du. And cos(4x) isn't correct. Remember the chain rule.

    Again you mean dy/dx and the derivative isn't correct. And where did the + c come from? You aren't integrating.

    Finally, once you correct the derivatives you need to write something like this:

    [tex]lim_{x\rightarrow 0}\frac {\sin(4x)}{x^2+8x} = \ ?[/tex]

    and write on the right side what you get.
  4. May 5, 2010 #3


    Staff: Mentor

    This is not right. You forgot to use the chain rule, plus it's missing the dx on the right side.
    This is also incorrect. The derivative of 8x is 8, not c. You're also missing the dx.

    In any case, you need to be working with derivatives, not differentials.

    What do you get when you actually apply L'Hopital's Rule? This rule says that under certain conditions,
    [tex]\lim_{x \to a} \frac{f(x)}{g(x)} = \lim_{x \to a} \frac{f'(x)}{g'(x)} [/tex]
    If the latter limit exists, the first limit is equal to it.
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