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Homework Help: Is f(x) differentiable at x=1?

  1. Jul 6, 2010 #1
    http://www.mathhelpforum.com/math-help/vlatex/pics/105_fde5ac6b051b4fac473487c7b4afa9e5.png [Broken]

    Is f(x) differentiable at x=1?

    I know that we have to prove
    http://www.mathhelpforum.com/math-help/vlatex/pics/65_6fae3c52eaa96aaafdf2c225a900ea48.png [Broken]

    exist/does not exist at x=1. But how do I begin? It's a piece-wise function.

    Thanks for your help.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 6, 2010 #2


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    Start by finding out the reasons that limit might not exist. This is a common type of problem, so you should find examples in your textbook.
  4. Jul 6, 2010 #3


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    The idea is one sided limits. You can compute the derivatives of f for x>1, and x<=1, for the derivative f' to exist, f' must be continuous there. So you've reduced the problem to continuity.
  5. Jul 6, 2010 #4


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    To add to hunt mat's response, while a derivative function is not necessarily continuous, it is continuous where ever it is defined. That is why taking the left and right side limits of the derivative works- if they are the same, the function is differentiable and the joint value is the derivative at that point.

    What is the derivative of [itex]x^2[/itex]? What is its value at x= 1? What is the derivative of (x+ 1)/2? what is its value at x= 1?
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