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Math Help - derivative: can't get rid of 1/h

  1. #1
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    derivative: can't get rid of 1/h

    Hi. I'm trying to review stuff I learn in Calc I, and I'm up to derivatives. However, I'm stumped on this prob from my old book:

    find derivative of 1/sqrt(x+2)

    After putting it in the form lim(h->0) [(1/h)(f(x+h)-f(x))], substituting, and doing some algebra I end up with

    lim(h->0)[(1/h)((sqrt(x+2)-sqrt(x+h+2))/(sqrt(x+h+2)*sqrt(x+2)))]

    If I multiple by conjugate I get 2x+h+4 on the top, and something really compilicated on the bottom, and I'm still not clear on how to get rid of the 1/h. Please advise me on the correct approach.
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  2. #2
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    Re: derivative: can't get rid of 1/h

    Quote Originally Posted by infraRed View Post
    Hi. I'm trying to review stuff I learn in Calc I, and I'm up to derivatives. However, I'm stumped on this prob from my old book:

    find derivative of 1/sqrt(x+2)

    After putting it in the form lim(h->0) [(1/h)(f(x+h)-f(x))], substituting, and doing some algebra I end up with

    lim(h->0)[(1/h)((sqrt(x+2)-sqrt(x+h+2))/(sqrt(x+h+2)*sqrt(x+2)))]

    If I multiple by conjugate I get 2x+h+4 on the top, and something really compilicated on the bottom, and I'm still not clear on how to get rid of the 1/h. Please advise me on the correct approach.
    You don't get that if you multiply correctly! [tex](\sqrt{x+2}- \sqrt{x+ h+ 2})(\sqrt{x+ 2}+ \sqrt{x+ h+ 2})= (x+ 2)- (x+ h+ 2)= -h. You added instead of subtracting.

    The denominator will be (\sqrt{x+ 2}+ \sqrt{x+ h+ 2})\sqrt{x+ 2}{\sqrt{x+h+ 2}. Yes, that's complicated but now you can let h go to 0.
    Thanks from infraRed
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    Re: derivative: can't get rid of 1/h

    Oops. Duh! I can't count the number of times a mishandled sign has cost me half-an-hour in homework time. Thanks for the help!
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    Re: derivative: can't get rid of 1/h

    Quote Originally Posted by infraRed View Post
    Oops. Duh! I can't count the number of times a mishandled sign has cost me half-an-hour in homework time. Thanks for the help!
    You're not alone.

    - Hollywood
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