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Homework Help: First principles

  1. Jun 2, 2008 #1
    [SOLVED] First principles

    Determine the derivatives of the following function from first principles (i.e using the limit
    definition of a derivative).

    g(x) = x^(3/2)

    lim h->0 : (x^(3/2) - (x + h)^(3/2) ) / (-h)

    I understand first principles its more so the expansion of the (x + h)^(3/2) that has got me lost.
  2. jcsd
  3. Jun 2, 2008 #2


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    Use the binomial theorem for non-integer powers. Then expand it for the first few terms, neglect powers of h by truncating the expansion (justified because h approaches zero) then substitute this result into the limit. Something will cancel out and you'll get the answer.
  4. Jun 2, 2008 #3
    (x + h)^(3/2)

    => ((x + h)^3))^(1/2)

    (x^3 + 2hx^2 + x(h^2) + h(x^2) + 2x(h^2) + h^3)^(1/2)

    So i get the following

    (x^(3/2) - (x^3 + 2hx^2 + x(h^2) + h(x^2) + 2x(h^2) + h^3)^(1/2)) / (-h)

    Firstly is this correct?
    Secondly what do i do now? take out a factor form the square root, but what factor? >.<
  5. Jun 2, 2008 #4

    Gib Z

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    The binomial theorem for non-integer powers is a bit of a deeper result than the derivative of a monomial isn't it?

    Why not take this approach:

    [tex] \frac{ (x+h)^{\frac{3}{2}} - x^{\frac{3}{2}} }{h} = \frac{ (x+h) \sqrt{x+h} - x\sqrt{x} }{h} = \frac{ x\sqrt{x+h} + h\sqrt{x+h} - x\sqrt{x}}{h} [/tex]

    Perhaps you can take it from here =]
  6. Jun 2, 2008 #5
    Thank you Gib Z!
    As i said i was just having problems with the power 1.5 but after seeing it written like that made everything much clearer.

    I multiplied by the conjugate and as a result canceled the h term from the denominator and got the proper result (3/2)x^(1/2).

  7. Jun 3, 2008 #6

    Gib Z

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    Good work, and no problem =] Just in case you are wondering, Defennder's suggestion works as well, though it may be beyond you at the moment.
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