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Proof: Differentiability implies continuity

If a function is differentiable then it's also continuous. This property is very useful when working with functions, because if we know that a function is differentiable, we immediately know that it's also continuous.
The AP Calculus course doesn't require knowing the proof of this fact, but we believe that as long as a proof is accessible, there's always something to learn from it. In general, it's always good to require some kind of proof or justification for the theorems you learn.
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Proof: Differentiability implies continuitySee video transcript

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  • male robot hal style avatar for user Arghya Roy
    I'm just wondering, at why did Sal choose the lim x->c(f(x)-f(c))? Was it just an arbitrary limit that he chose?
    Thanks
    (26 votes)
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  • blobby green style avatar for user Vesselin Atanasov
    Arguably this is the same proof formatted in a more intuitive way:

    (1) lim{x->c}{(f(x)-f(c))/(x-c)} = f'(c)
    (2) lim{x->c}{(f(x)-f(c))/(x-c)} * lim{x->c}{x-c} = f'(c) * lim{x->c}{x-c}
    (3) lim{x->c}{x-c} = 0
    From (2) and (3) we have
    (4) lim{x->c}{(f(x)-f(c))/(x-c)} * lim{x->c}{x-c} = f'(c) * 0
    From (4) and the rule about limit multiplication we have
    (5) lim{x->c}{((f(x)-f(c))/(x-c)) * (x-c)} = 0
    (6) lim{x->c}{f(x)-f(c)} = 0
    (7) lim{x->c}{f(x)} - lim{x->c}{f(c)} = 0
    (8) lim{x->c}{f(x)} - f(c) = 0
    (9) lim{x->c}{f(x)} = f(c)
    (19 votes)
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    • starky seedling style avatar for user deka
      i also may vote for this direction than the one used in video



      by the assumption of a functions' differentiability we can start with (1) then drive toward (9)

      but in video, Sal dives into the proof from (6) then goes to both directions, up and down, to get the "intended" match

      but we can't talk about (6) only with its differentiability. in fact, (6) is what we're going to prove with (1). thus it feels like a circular reasoning



      the core concepts used in both proofs are the same [lim(A*B) = lim(A)*lim(B) and lim(A+B) = lim(A)+lim(B)]. the difference is in flows. and i bet this one is right
      (0 votes)
  • leafers seed style avatar for user bogaofspring
    i love you Sal, ur the best for real
    (10 votes)
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  • blobby green style avatar for user Harikesh
    At Why did we assume that lim x -> c f(c) = f(c) ?

    What if function f(x) is undefined at c or if lim x -> c f(c) does not exist ?
    (1 vote)
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  • starky sapling style avatar for user p h
    HELP! I've been thinking about this topic for a little bit. I've watched the video a couple of times and consulted a few other sources, and I can say that I am thoroughly confused.

    How exactly does this proof show that Differentiability implies continuity? I can follow the math. But, I think that I'm missing something here. What are the key parts of this proof and how does it link together the topics of differentiability and continuity?
    (2 votes)
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    • duskpin sapling style avatar for user Leeco Gutierrez
      This proves that differentiability implies continuity when we look at the equation Sal arrives to at . If the derivative does not exist, then you end up multiplying 0 by some undefined, which is nonsensical. If the derivative does exist though, we end up multiplying a 0 by f'(c), which allows us to carry on with the proof.
      (14 votes)
  • purple pi purple style avatar for user Suvro Banerjee
    Just curious to know how others interpret ?
    Intuitively why limit x -> c (f(x) - f(c)) = 0. I understand the math but want to know what does that mean, and why it becomes 0 intuitively ? Thanks.
    (2 votes)
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  • mr pink orange style avatar for user Steve L
    I am trying to understand what “Differentiability Implies Continuity” means. I made the incorrect assumption that to prove differentiability, all I needed to do was confirm that:
    lim x->c- (f(x) – f(c))/(x – c) = lim x->c+ (f(x) – f(c))/(x – c)
    The assumption was incorrect. All the above indicates is that the slope as ‘x’ approaches ‘c’ from the left and right is the same. It does not indicate that the function is continuous at ‘c’, i.e. lim x->c f(x) = f(c). It seems that to prove differentiability, it is necessary to first prove continuity; which seems counter to the statement that “Differentiability Implies Continuity”. What am I missing?
    (0 votes)
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    • duskpin sapling style avatar for user Melissa L.
      I think your assumption, "lim x->c- (f(x) – f(c))/(x – c) = lim x->c+ (f(x) – f(c))/(x – c)" implies that "continuity implies differentiability". However, Khan showed examples of how there are continuous functions which have points that are not differentiable. For example, f(x)=absolute value(x) is continuous at the point x=0 but it is NOT differentiable there. In addition, a function is NOT differentiable if the function is NOT continuous. In this video, Khan is merely proving that if you know the function is differentiable, then it MUST also be continuous for all the points at which it is differentiable.
      (10 votes)
  • blobby green style avatar for user Nice Guy
    By using "implies" does this mean that where a function is differentiable the function must be continuous or does that mean that it strongly indicates that the function is also continuous? (assuming the must)
    (2 votes)
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  • blobby green style avatar for user Hanwei Zhou
    at is the sentence "the function f(x) is differentiable at x=c... really just saying this llimit right over here actually exists" is it indicates that if the function f(x) is differentiable at x=c, then the limit exists and if the limit exists, then the function f(x) is differentiable at x=c, which is known as biconditional statement(iff)?
    (1 vote)
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    • blobby green style avatar for user raviteja vaddadi
      It is not bi-directional,
      In case if f(x) is differentiable at x=c, then limit exist, yes and and f(x) is continuous at x(proved in the above theorom).
      And in case if f(x) is said to be continuous, we can't simply say it is differentiable, since its not just limit of f(x) we are calculating but limit of slope of f(x) is what we need to calculate to find differentiability property of f(x).
      Hope this helpful.
      (2 votes)
  • leaf green style avatar for user Lio
    So how to justify though f(x) is continous, it doesn't mean derivative
    (1 vote)
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