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Question about determining whether to use the chain rule or not?

  1. Feb 21, 2014 #1
    For example,

    if you differentiate 6*sqrt(x^5), would you use the chain rule? If not, why?

    Thank you!
     
  2. jcsd
  3. Feb 21, 2014 #2

    Mark44

    Staff: Mentor

    Should you use chain rule? It depends.

    As your function is written, you have a composite function (a function whose argument is another function). To differentiate such a function requires the chain rule.

    If you write your function as 6x5/2, though, now it's no longer a composite, so you could use the power rule (and also the constant multiple rule).
     
  4. Feb 22, 2014 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    It's not a matter of applying some "hard and fast" rule. You use your knowledge and think about each individual problem. Any time you can see something that can be thought of as a composition of two (or more) functions, that is candidate for the chain rule. To differentiate [itex]6\sqrt{x^5}[/itex] you can think of its as f(g(x)) where [itex]f(x)= 6\sqrt{x}[/itex] and [itex]g(x)= x^5[/itex].

    In that case, [itex]g'(x)= 5x^4[/itex] and [itex]f'(x)= (6x^{1/2})'= 6(1/2)x^{-1/2}= 3/\sqrt{x}[/itex] so the derivative is [itex](3/\sqrt{x^5})(5x^4)= 15(x^{-5/2})(x^4)[/itex][itex]= 15x^{-5/2+ 4}= 15x^{3/2}[/itex].


    But, in this particular case, it is easier to do as Mark44 suggested: write the function as [itex]6(x^5)^{1/2}= 6x^{5/2}[/itex] so its derivative is [itex]6(5/2)x^{5/2- 1}= 15x^{3/2}[/itex] as above
     
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