Proof of L'Hopital's rule ($x_0 notin D$)












0












$begingroup$


Is this a rigorous proof for Hospital rule? (case $lim_{x to x_0} f(x) = g(x) = 0$)



We suppose that $x_0$ doesn't belong to the domain of $f(x),g(x)$, but it is a limit point.



Since $x_0$ is a limit point for $f(x),f(x)$, there is a sequence $x_n: lim_{n to infty} x_n = x_0$.



$frac{f(x) - f(x_n)}{g(x) - g(x_n)} = frac{f'(a_n)}{g'(a_n)}$ $quad$ Cauchy's mean value theorem (with $a_n in (x,x_n) ,,forall , n$)



If we let $n to infty$ we get



$frac{f(x) - 0}{g(x) - 0)} = frac{f(x)}{g(x)} = frac{f'(a_n)}{g'(a_n)}$ $quad a_n in (x,x_0)$



$lim_{x to x_0} frac{f(x)}{g(x)} = frac{f'(x_0)}{g'(x_0)}$ $quad$ because $a_n$ is "squeezed" between $x$ and $x_0$, and $x$ is going to $x_0$










share|cite|improve this question











$endgroup$












  • $begingroup$
    You probably mean L'Hopital's rule
    $endgroup$
    – roman
    Jan 3 at 8:39










  • $begingroup$
    @roman: L'Hôpital's rule, to be precise :)
    $endgroup$
    – Martin R
    Jan 3 at 8:43










  • $begingroup$
    How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
    $endgroup$
    – 5xum
    Jan 3 at 8:47










  • $begingroup$
    @5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:50


















0












$begingroup$


Is this a rigorous proof for Hospital rule? (case $lim_{x to x_0} f(x) = g(x) = 0$)



We suppose that $x_0$ doesn't belong to the domain of $f(x),g(x)$, but it is a limit point.



Since $x_0$ is a limit point for $f(x),f(x)$, there is a sequence $x_n: lim_{n to infty} x_n = x_0$.



$frac{f(x) - f(x_n)}{g(x) - g(x_n)} = frac{f'(a_n)}{g'(a_n)}$ $quad$ Cauchy's mean value theorem (with $a_n in (x,x_n) ,,forall , n$)



If we let $n to infty$ we get



$frac{f(x) - 0}{g(x) - 0)} = frac{f(x)}{g(x)} = frac{f'(a_n)}{g'(a_n)}$ $quad a_n in (x,x_0)$



$lim_{x to x_0} frac{f(x)}{g(x)} = frac{f'(x_0)}{g'(x_0)}$ $quad$ because $a_n$ is "squeezed" between $x$ and $x_0$, and $x$ is going to $x_0$










share|cite|improve this question











$endgroup$












  • $begingroup$
    You probably mean L'Hopital's rule
    $endgroup$
    – roman
    Jan 3 at 8:39










  • $begingroup$
    @roman: L'Hôpital's rule, to be precise :)
    $endgroup$
    – Martin R
    Jan 3 at 8:43










  • $begingroup$
    How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
    $endgroup$
    – 5xum
    Jan 3 at 8:47










  • $begingroup$
    @5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:50
















0












0








0





$begingroup$


Is this a rigorous proof for Hospital rule? (case $lim_{x to x_0} f(x) = g(x) = 0$)



We suppose that $x_0$ doesn't belong to the domain of $f(x),g(x)$, but it is a limit point.



Since $x_0$ is a limit point for $f(x),f(x)$, there is a sequence $x_n: lim_{n to infty} x_n = x_0$.



$frac{f(x) - f(x_n)}{g(x) - g(x_n)} = frac{f'(a_n)}{g'(a_n)}$ $quad$ Cauchy's mean value theorem (with $a_n in (x,x_n) ,,forall , n$)



If we let $n to infty$ we get



$frac{f(x) - 0}{g(x) - 0)} = frac{f(x)}{g(x)} = frac{f'(a_n)}{g'(a_n)}$ $quad a_n in (x,x_0)$



$lim_{x to x_0} frac{f(x)}{g(x)} = frac{f'(x_0)}{g'(x_0)}$ $quad$ because $a_n$ is "squeezed" between $x$ and $x_0$, and $x$ is going to $x_0$










share|cite|improve this question











$endgroup$




Is this a rigorous proof for Hospital rule? (case $lim_{x to x_0} f(x) = g(x) = 0$)



We suppose that $x_0$ doesn't belong to the domain of $f(x),g(x)$, but it is a limit point.



Since $x_0$ is a limit point for $f(x),f(x)$, there is a sequence $x_n: lim_{n to infty} x_n = x_0$.



$frac{f(x) - f(x_n)}{g(x) - g(x_n)} = frac{f'(a_n)}{g'(a_n)}$ $quad$ Cauchy's mean value theorem (with $a_n in (x,x_n) ,,forall , n$)



If we let $n to infty$ we get



$frac{f(x) - 0}{g(x) - 0)} = frac{f(x)}{g(x)} = frac{f'(a_n)}{g'(a_n)}$ $quad a_n in (x,x_0)$



$lim_{x to x_0} frac{f(x)}{g(x)} = frac{f'(x_0)}{g'(x_0)}$ $quad$ because $a_n$ is "squeezed" between $x$ and $x_0$, and $x$ is going to $x_0$







calculus limits derivatives






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share|cite|improve this question













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share|cite|improve this question








edited Jan 3 at 8:42







Francesco Andreuzzi

















asked Jan 3 at 8:36









Francesco AndreuzziFrancesco Andreuzzi

63




63












  • $begingroup$
    You probably mean L'Hopital's rule
    $endgroup$
    – roman
    Jan 3 at 8:39










  • $begingroup$
    @roman: L'Hôpital's rule, to be precise :)
    $endgroup$
    – Martin R
    Jan 3 at 8:43










  • $begingroup$
    How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
    $endgroup$
    – 5xum
    Jan 3 at 8:47










  • $begingroup$
    @5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:50




















  • $begingroup$
    You probably mean L'Hopital's rule
    $endgroup$
    – roman
    Jan 3 at 8:39










  • $begingroup$
    @roman: L'Hôpital's rule, to be precise :)
    $endgroup$
    – Martin R
    Jan 3 at 8:43










  • $begingroup$
    How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
    $endgroup$
    – 5xum
    Jan 3 at 8:47










  • $begingroup$
    @5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:50


















$begingroup$
You probably mean L'Hopital's rule
$endgroup$
– roman
Jan 3 at 8:39




$begingroup$
You probably mean L'Hopital's rule
$endgroup$
– roman
Jan 3 at 8:39












$begingroup$
@roman: L'Hôpital's rule, to be precise :)
$endgroup$
– Martin R
Jan 3 at 8:43




$begingroup$
@roman: L'Hôpital's rule, to be precise :)
$endgroup$
– Martin R
Jan 3 at 8:43












$begingroup$
How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
$endgroup$
– 5xum
Jan 3 at 8:47




$begingroup$
How do you know that $x_n>x$? If you don't know that, then how do you know $(x,x_n)$ is not an empty set?
$endgroup$
– 5xum
Jan 3 at 8:47












$begingroup$
@5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:50






$begingroup$
@5xum isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:50












1 Answer
1






active

oldest

votes


















1












$begingroup$

The proof is a fine idea, but it is not a rigorous proof as it has some hand-waving and sloppiness in it. Two issues are:




  1. You say that $a_n=(x,x_n)$ for all $n$, however there is no reason for a reader at that point to assume that $x_n>x$, and therefore, the interval $(x,x_n)$ might be empty.

  2. You say that you "let $ntoinfty$", and get $frac{f(x)}{g(x)}=frac{f'(a_n)}{g'(a_n)}$. There are two issues here. One is that saying "let $ntoinfty$" is not particularly rigorous in itself. Two, if you sent $n$ to $infty$, then how can you get an equality which still depends on $n$?






share|cite|improve this answer









$endgroup$













  • $begingroup$
    1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:53










  • $begingroup$
    @FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
    $endgroup$
    – 5xum
    Jan 3 at 8:54










  • $begingroup$
    2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:55










  • $begingroup$
    @FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
    $endgroup$
    – 5xum
    Jan 3 at 8:56






  • 1




    $begingroup$
    @FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
    $endgroup$
    – 5xum
    Jan 3 at 9:03












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1 Answer
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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1












$begingroup$

The proof is a fine idea, but it is not a rigorous proof as it has some hand-waving and sloppiness in it. Two issues are:




  1. You say that $a_n=(x,x_n)$ for all $n$, however there is no reason for a reader at that point to assume that $x_n>x$, and therefore, the interval $(x,x_n)$ might be empty.

  2. You say that you "let $ntoinfty$", and get $frac{f(x)}{g(x)}=frac{f'(a_n)}{g'(a_n)}$. There are two issues here. One is that saying "let $ntoinfty$" is not particularly rigorous in itself. Two, if you sent $n$ to $infty$, then how can you get an equality which still depends on $n$?






share|cite|improve this answer









$endgroup$













  • $begingroup$
    1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:53










  • $begingroup$
    @FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
    $endgroup$
    – 5xum
    Jan 3 at 8:54










  • $begingroup$
    2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:55










  • $begingroup$
    @FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
    $endgroup$
    – 5xum
    Jan 3 at 8:56






  • 1




    $begingroup$
    @FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
    $endgroup$
    – 5xum
    Jan 3 at 9:03
















1












$begingroup$

The proof is a fine idea, but it is not a rigorous proof as it has some hand-waving and sloppiness in it. Two issues are:




  1. You say that $a_n=(x,x_n)$ for all $n$, however there is no reason for a reader at that point to assume that $x_n>x$, and therefore, the interval $(x,x_n)$ might be empty.

  2. You say that you "let $ntoinfty$", and get $frac{f(x)}{g(x)}=frac{f'(a_n)}{g'(a_n)}$. There are two issues here. One is that saying "let $ntoinfty$" is not particularly rigorous in itself. Two, if you sent $n$ to $infty$, then how can you get an equality which still depends on $n$?






share|cite|improve this answer









$endgroup$













  • $begingroup$
    1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:53










  • $begingroup$
    @FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
    $endgroup$
    – 5xum
    Jan 3 at 8:54










  • $begingroup$
    2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:55










  • $begingroup$
    @FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
    $endgroup$
    – 5xum
    Jan 3 at 8:56






  • 1




    $begingroup$
    @FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
    $endgroup$
    – 5xum
    Jan 3 at 9:03














1












1








1





$begingroup$

The proof is a fine idea, but it is not a rigorous proof as it has some hand-waving and sloppiness in it. Two issues are:




  1. You say that $a_n=(x,x_n)$ for all $n$, however there is no reason for a reader at that point to assume that $x_n>x$, and therefore, the interval $(x,x_n)$ might be empty.

  2. You say that you "let $ntoinfty$", and get $frac{f(x)}{g(x)}=frac{f'(a_n)}{g'(a_n)}$. There are two issues here. One is that saying "let $ntoinfty$" is not particularly rigorous in itself. Two, if you sent $n$ to $infty$, then how can you get an equality which still depends on $n$?






share|cite|improve this answer









$endgroup$



The proof is a fine idea, but it is not a rigorous proof as it has some hand-waving and sloppiness in it. Two issues are:




  1. You say that $a_n=(x,x_n)$ for all $n$, however there is no reason for a reader at that point to assume that $x_n>x$, and therefore, the interval $(x,x_n)$ might be empty.

  2. You say that you "let $ntoinfty$", and get $frac{f(x)}{g(x)}=frac{f'(a_n)}{g'(a_n)}$. There are two issues here. One is that saying "let $ntoinfty$" is not particularly rigorous in itself. Two, if you sent $n$ to $infty$, then how can you get an equality which still depends on $n$?







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 3 at 8:51









5xum5xum

91.8k394161




91.8k394161












  • $begingroup$
    1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:53










  • $begingroup$
    @FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
    $endgroup$
    – 5xum
    Jan 3 at 8:54










  • $begingroup$
    2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:55










  • $begingroup$
    @FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
    $endgroup$
    – 5xum
    Jan 3 at 8:56






  • 1




    $begingroup$
    @FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
    $endgroup$
    – 5xum
    Jan 3 at 9:03


















  • $begingroup$
    1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:53










  • $begingroup$
    @FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
    $endgroup$
    – 5xum
    Jan 3 at 8:54










  • $begingroup$
    2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
    $endgroup$
    – Francesco Andreuzzi
    Jan 3 at 8:55










  • $begingroup$
    @FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
    $endgroup$
    – 5xum
    Jan 3 at 8:56






  • 1




    $begingroup$
    @FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
    $endgroup$
    – 5xum
    Jan 3 at 9:03
















$begingroup$
1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:53




$begingroup$
1. isn't $x$ an arbitrary value? Since $x_n$ converges, it's also bounded, and so I could choose $x leq inf x_n$
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:53












$begingroup$
@FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
$endgroup$
– 5xum
Jan 3 at 8:54




$begingroup$
@FrancescoAndreuzzi Well I don't know what $x$ is, to be honest. A more rigorous write up might help you avoid such unclarities.
$endgroup$
– 5xum
Jan 3 at 8:54












$begingroup$
2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:55




$begingroup$
2. I don't know if $a_n$ converges, and if yes, what's the limit, so I left $a_n$ in the equality as it was. I didn't know that this could be a problem
$endgroup$
– Francesco Andreuzzi
Jan 3 at 8:55












$begingroup$
@FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
$endgroup$
– 5xum
Jan 3 at 8:56




$begingroup$
@FrancescoAndreuzzi I mean, if we can just change $x$, then you didn't even define $a_n$ rigorously enough. LIke I am saying, your proof has a good idea, but it is not rigorous. I would expect a rigorous proof to include some $epsilon-delta$ definitions, or references to other theorems.
$endgroup$
– 5xum
Jan 3 at 8:56




1




1




$begingroup$
@FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
$endgroup$
– 5xum
Jan 3 at 9:03




$begingroup$
@FrancescoAndreuzzi LIke I said, it's not that you made a mistake, it's more that your proof is not finished yet. It's an idea of a proof. It's not a rigorous proof yet.
$endgroup$
– 5xum
Jan 3 at 9:03


















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