March 28, 2017

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find derivative using limit definition:

f(x) = x - sqrt(x)

so f'(x) =
h->0 [f(x+h) - f(x)]/h

but I keep trying to solve by multiplying by the conjugate but I can't figure it out..there's nothing that can be cancelled or anything and I can't get the derivative

sorry this is a repost, but i messed up my other one and i really need help on this

  • calc - ,

    The key to this is the binomial theorem:

    (x+h)^n = x^n + n*x^(n-1)*h + n(n-1)/2 * x^(n-2) * h^2 + ...

    So, we find ourselves with

    (h + sqrt(x) - sqrt(x+h))/h
    = (h + x^(1/2) - (x^(1/2) + 1/2 * x^(-1/2)*h + <higher powers of h>)/h
    = 1 - 1/2 * x^(-1/2)
    (all terms with h go to zero)

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