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Math Help - Chain rule problem

  1. #1
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    Chain rule problem

    Hey, I have a question that I do not know how to go about answering.

    Question:
    Let  y = g(h(x)) where  h(x) = \frac{x^2}{x+2} . If  g'(9/5) = -2 , find dy/dx when  x = 3

    Attempted answer:
    Basically I have found the derivative of  h(x) = \frac{x^2}{x+2} . Which is  h'(x) = \frac {-x^2}{(x + 2)^2} + \frac {2x}{(x+2)}

    I do not know what I am supposed to do with  g'(9/5) = -2 . How do I find what the function g(x) is from that?

    Thanks!
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  2. #2
    Ted
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    Quote Originally Posted by Kakariki View Post
    Hey, I have a question that I do not know how to go about answering.

    Question:
    Let  y = g(h(x)) where  h(x) = \frac{x^2}{x+2} . If  g'(9/5) = -2 , find dy/dx when  x = 3

    Attempted answer:
    Basically I have found the derivative of  h(x) = \frac{x^2}{x+2} . Which is  h'(x) = \frac {-x^2}{(x + 2)^2} + \frac {2x}{(x+2)}

    I do not know what I am supposed to do with  g'(9/5) = -2 . How do I find what the function g(x) is from that?

    Thanks!
    Start by this:
    If y=g\left(h(x)\right) then y'=g'\left(h(x)\right) h'(x).
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  3. #3
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    Thanks, that's the chain rule. I have figured out that g'(9/5) basically is the same thing as  \frac {(3)^2}{(3) + 2)} (subbing 3 in for x). What does this tell me? I still do not understand how to find what g(x) is.

    Basically asking for a more detailed explanation.

    Thanks for the response though!
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  4. #4
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    Quote Originally Posted by Kakariki View Post
    Thanks, that's the chain rule. I have figured out that g'(9/5) basically is the same thing as  \frac {(3)^2}{(3) + 2)} (subbing 3 in for x). What does this tell me? I still do not understand how to find what g(x) is.
    It tells you the answer! The problem says NOTHING about finding g(x) and there is no reason to do that. f(3)= 9/5 so the derivative of g(f(x)), at x= 3 is g'(9/5)f'(3).

    Basically asking for a more detailed explanation.

    Thanks for the response though!
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