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Math Help - finding f''(x)

  1. #1
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    Jan 2011
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    finding f''(x)

    x^2 + xy = 5,

    I figured out that f'(x)= (-2x-y)/x, and that f''(x)= (y-xy')/x^2.

    I plug-in f'(x) into f''(x) and I get, y-x(-2x-y)/x^3, and I simplified it to (y+2x^2+xy)/x^3. rewriting it as (y+2(5))/x^3, then (y+10)/x^3

    The back of the book says it is supposed to be just 10/x^3, where is this extra y coming from?
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  2. #2
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    actually nevermind, I figured out where i went wrong.
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