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Implicit Differentiation and the Chain Rule

  1. Jun 26, 2013 #1
    Hi,

    I was trying to understand why the chain rule is needed to differentiate expressions implicitly.

    I began by analyzing the equation used by most websites I visited:

    e.g. x2+y2 = 10

    After a lot of thinking, I got to a reasoning that satisfied me... Here it goes:

    From my understanding, the variable y is a function of x. This function of x is being squared. This means that we can think of f(x) as part of another function (e.g. u = g(y) = y^2). Hence, y^2 is a composite function and, thus, differentiating it would require the chain rule.

    However, after coming across some different type of questions I am no longer sure my train of thought is valid. For example:

    6x^2+17y = 0.

    I have read that to differentiate 17 y with respect to x we also have to apply the chain rule. This does not fit with my original reasoning (since, to my eyes, y cannot be thought of as a composite function in this case)

    Can anyone please help me understand why we have to use the chain rule to differentiatie implicitly?

    Thank you in advance!
     
  2. jcsd
  3. Jun 26, 2013 #2

    Mark44

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    Staff: Mentor

    No need to apply the chain rule here. You're just differentiating y with respect to x (to get dy/dx). In your previous example, you did need to use the chain rule, since d/dx(y2) = d/dy(y2) * dy/dx. Here we had a function of y, and y itself was a function of x.
     
  4. Jun 26, 2013 #3
    Oh, perfect, that is great news! I must have misread the solution to the last problem. Thank you very much! :smile:
     
  5. Jun 27, 2013 #4
    You can think of y as a composite function, if you think of x as a function of x.
     
  6. Jun 27, 2013 #5
    Technically you do use the chain rule, because 17y is a function of y.
     
  7. Jun 27, 2013 #6

    pasmith

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    Homework Helper

    Or by using the product rule, together with the fact that
    [tex]
    \frac{\mathrm{d}(17)}{\mathrm{d}x} = 0.
    [/tex]
     
  8. Jun 27, 2013 #7

    Mark44

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    Staff: Mentor

    Just because 17y is a function of y doesn't mean that the chain rule needs to be used.
    Or better yet, the constant multiple rule. d/dx(17y) = 17 * dy/dx.
     
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