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Homework Help: How does the following simplify?

  1. Oct 23, 2013 #1
    1. The problem statement

    ( F(r) [itex]\frac{d}{dr}[/itex] ( [itex]r^{2}[/itex] [itex]\frac{d}{dr}[/itex] ) ) g(r)

    2. The attempt at a solution

    Does it just melt down to:

    ( F(r) [itex]r^{2}[/itex] g(r) ) [itex]\frac{d^{2}}{dr^{2}}[/itex]
  2. jcsd
  3. Oct 23, 2013 #2


    Staff: Mentor

    It pretty much doesn't make sense at all. Except in the context of operators, which I don't think is applicable here, d/dr and d2/dr2 don't stand on their own.

    The derivative operator d/dr should be applied to some function of r, such as d/dr(r2), which simplifies to 2r.
  4. Oct 23, 2013 #3


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    To add to what Mark44 said, presumably you mean [itex]F(r) \frac{d}{dr}[/itex] ( [itex]r^{2}[/itex] [itex]\frac{d}{dr}g(r)[/itex] ). You can expand the outer d/dr using the product rule.
  5. Oct 24, 2013 #4
    sure, I don't trust operators so forgive me but I have to ask if I can presume that the order of working through such an example might then follows something like this...

    k(r) = [itex]r^{2}[/itex]

    H(r) = [itex]\frac{d}{dr}[/itex] ( k(r) g(r) ) ------> using the product rule

    => [itex]\frac{d}{dr}[/itex] ( F(r) H(r) ) ------> using the product rule...

    or this:

    => F(r) x { [itex]\frac{d}{dr}[/itex] [ [itex]r^{2}[/itex] x [itex]\frac{dg(r)}{dr}[/itex] ] }, using the product rule for the parts in the [] brackets
  6. Oct 24, 2013 #5


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    I see no way to move the F(r) inside the derivative like that. It certainly is not the product rule.
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