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Help on another proof that differentiability implies continous parital derivatives

  1. May 11, 2009 #1
    Ok, so I have f(x,y)=(p(x)+q(y))/(x^2+y^2) where (x,y)NOT=0 and f(0,0)=0. the basic idea of the function is that the numerator contains 2 polynomials>2nd order. and the denominator has a Xsquared+ysquared. I have to prove that if f(x,y) is differentiable at (0,0) then its partial derivatives fx and fy are both continous. I need something rigorous i was thinking of doing something where the tangent h(x,y) approximates f(x,y) as dx, dy go to 0. and then from there.... IDK i need all help i can get. Thank you in advance
     
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  3. May 12, 2009 #2

    HallsofIvy

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    Re: help on another proof that differentiability implies continous parital derivative

    What is the definition of "differentiable" at a point, for functions of several variables?
     
  4. May 12, 2009 #3
    Re: help on another proof that differentiability implies continous parital derivative

    that the partial derivatives are continous in a neighborhood BUT that proves that if partial derivs are cont., then the function is differentiable. I need to prove that if its differentiable, the partials are continous
     
  5. May 13, 2009 #4

    HallsofIvy

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    Re: help on another proof that differentiability implies continous parital derivative

    Then I ask again, "What is the definition of "differentiable" at a point, for functions of several variables?:"

    A definition is always an "if and only if" statement so what you give is clearly NOT a definition.
     
  6. May 13, 2009 #5
    Re: help on another proof that differentiability implies continous parital derivative

    hmmm. A function is differentiable at a point if and only if Fx and Fy are continous? i still dont know where to start the proof for the generic problem though.
     
    Last edited: May 13, 2009
  7. May 13, 2009 #6
    Re: help on another proof that differentiability implies continous parital derivative

    In part B of this question i proved that if Fx and Fy are continous then f(x,y) is differentiable becuase a tangent plane exists there and i used lim at (0,0) (h(x,y)-f(x,y))/sqroot(x^2+Y^2) goes to 0 to complete the proof that it is differentiable. I don't know why I can't go the other way.
     
    Last edited: May 13, 2009
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