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Homework Help: Proof involving derivatives

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data

    Let the symmetric derivative of f at x be

    lim h->0 f(x+h) + f(x-h) - 2f(x) / h, provided the limit exists.

    Prove there exists a point, x, in (0,1) where the ordinary derivative exists.

    Note: f is cont. on [0,1], and the symm. deriv. exists everywhere on (0,1). Prove there is a point, x, in (0,1) where the ordinary derivative exists. Assume f(0)=f(1)=0 and it is unknown if f is differentiable.

    2. Relevant equations

    3. The attempt at a solution

    I don't know where to start on this. I tried MVT and Rolle's Thm, but unfortunately they don't seem to apply here. Also tried setting h(x) = f(x) - f(0) - x[f(1)-f(0)] and letting g(1)=g(0)=0, but no progress there either. Thx for any help.
  2. jcsd
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