Chain Rule

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  • #1
physicskid
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In finding a derivative of a value, how do you know whether when to use the power rule or the chain rule? can anyone please tell me?
 

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  • #2
Gza
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Usually any good calc textbook will walk you through the process. It's rather mechanical. If you have a function inside of another function, you differentiate the outside one and multiply it by the inside one. With the power rule, you are still using the chain rule without knowing it. For example, in differentiating the function
f(x) = x^2, you take the derivative of the "outside" (x^2) times the derivative of the "inside" (x) yielding f(x) = 2x*(1) . You multiply by one since the chain rule told you to multiply by the derivative of the inside function. I'm not sure if I answered your question, but you'd be better off taking a peek at a good calc textbook (try Stewarts), and working a few problems until you get it. It's really a skill you need to practice to understand how the rules apply.
 
  • #3
rcg
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If the equation is in the form f(x)=ax^n, then you can use the power rule. If it is a composite function of some form, you can use the chain rule to keep it simple. f(x)=a(x+3)^n can be expanded out and differentiated with the power rule, but it's much easier to use chain.
Although I believe that the power rule is more a derivation of first principles as opposed to application of the chain rule, you can use chain if you want to...but it's so much easier to use anx^n-1 (Power rule).
 
  • #4
Nenad
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Its wery easy, there's no messing around. The power rule is basically the chain rule, but simpler and for easier derivatives.
Lets say we have a function
[tex] u(x) = kx^n [/tex]

[tex] \frac {d}{dx} u(x) = \frac {d(kx^n)}{dx} [/tex]

[tex] \frac {d}{dx} u(x) = (nk)x^{n-1} [/tex]

Now the chain rule. Let's say we have a function:

[tex] f(x) = (u(x))^n [/tex]

[tex] \frac {d}{dx} f(x) = \frac {d((u(x))^n)}{dx} [/tex]

[tex] \frac {d}{dx} f(x)= n(u(x))^n) \frac {d(u(x))}{dx} [/tex]

its a simple set of rules, the best way to get used to them is to practice different examples. Sorry if my notation at the end is a little funky, the latex notation is hard to work with.
 
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  • #5
mathwonk
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the chain rule works on the composition of any two functions at all f(g(x)).

the power rule is the special case where the outer function is a power (g(x))^n,

i.e. here u = g(x) is anything, but f(u) = u^n.
 

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