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Math Help - find the dy/dx

  1. #1
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    find the dy/dx

    if y=f(√(x+9) and f'(5)=-2, find dy/dx when x=4
    Last edited by hawtmonkey; February 26th 2009 at 10:41 PM.
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  2. #2
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    Quote Originally Posted by hawtmonkey View Post
    if y=f(√(x+9) and f'(5)=-2, find dy/dx when x=4
    Look at it like y=f(g[x]), where g(x) is the square root. Now just use the normal chain rule to find y'=f'(g[x])*g'[x]. Notice that g(4)=5 so that's why that extra bit was given to you. So now all you have to do is find g'[5].
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