Problem in evaluating logarithm derivatives












8












$begingroup$


Given the property of the logarithm that $log{xy} = log{x} + log{y}$, how would one take the 'derivative' of this?



To be more clear,



$log{xy} = log{x} + log{y}$ (property of $log$)



$D(log{xy}) = D(log{x} + log{y})$ (i) (Take derivative on both sides)



Now, $D(log{x} + log{y}) = D(log{x}) + D(log{y})$ (ii) (Derivative of sum is sum of derivatives)



Combining (i) and (ii): $D(log{xy}) = D(log{x}) + D(log{y})$ (iii)



Implies: $frac{1}{xy} = frac{1}{x} + frac{1}{y}$ (Evaluate derivative of logarithm using $D(log{x}) = frac{1}{x}$



$frac{1}{xy} = frac{1}{x} + frac{1}{y}$ looks false to me; e.g. while $log{6}$ does equal $log{2} + log{3}$, $frac{1}{6}$ does not equal $frac{1}{2} + frac{1}{3}$.



My first guess was that the issue was related to what variable I take the derivative with respect to, but I'd like to understand this a little more formally if someone could guide me.



What's going wrong in this example?










share|cite|improve this question











$endgroup$












  • $begingroup$
    Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
    $endgroup$
    – MathIsLife12
    Jan 5 at 4:19










  • $begingroup$
    (This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
    $endgroup$
    – Clement C.
    Jan 5 at 4:21












  • $begingroup$
    Thanks guys; this clears things up! I appreciate the fast responses.
    $endgroup$
    – user2192320
    Jan 5 at 4:24










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – John Doe
    Jan 5 at 5:01
















8












$begingroup$


Given the property of the logarithm that $log{xy} = log{x} + log{y}$, how would one take the 'derivative' of this?



To be more clear,



$log{xy} = log{x} + log{y}$ (property of $log$)



$D(log{xy}) = D(log{x} + log{y})$ (i) (Take derivative on both sides)



Now, $D(log{x} + log{y}) = D(log{x}) + D(log{y})$ (ii) (Derivative of sum is sum of derivatives)



Combining (i) and (ii): $D(log{xy}) = D(log{x}) + D(log{y})$ (iii)



Implies: $frac{1}{xy} = frac{1}{x} + frac{1}{y}$ (Evaluate derivative of logarithm using $D(log{x}) = frac{1}{x}$



$frac{1}{xy} = frac{1}{x} + frac{1}{y}$ looks false to me; e.g. while $log{6}$ does equal $log{2} + log{3}$, $frac{1}{6}$ does not equal $frac{1}{2} + frac{1}{3}$.



My first guess was that the issue was related to what variable I take the derivative with respect to, but I'd like to understand this a little more formally if someone could guide me.



What's going wrong in this example?










share|cite|improve this question











$endgroup$












  • $begingroup$
    Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
    $endgroup$
    – MathIsLife12
    Jan 5 at 4:19










  • $begingroup$
    (This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
    $endgroup$
    – Clement C.
    Jan 5 at 4:21












  • $begingroup$
    Thanks guys; this clears things up! I appreciate the fast responses.
    $endgroup$
    – user2192320
    Jan 5 at 4:24










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – John Doe
    Jan 5 at 5:01














8












8








8


1



$begingroup$


Given the property of the logarithm that $log{xy} = log{x} + log{y}$, how would one take the 'derivative' of this?



To be more clear,



$log{xy} = log{x} + log{y}$ (property of $log$)



$D(log{xy}) = D(log{x} + log{y})$ (i) (Take derivative on both sides)



Now, $D(log{x} + log{y}) = D(log{x}) + D(log{y})$ (ii) (Derivative of sum is sum of derivatives)



Combining (i) and (ii): $D(log{xy}) = D(log{x}) + D(log{y})$ (iii)



Implies: $frac{1}{xy} = frac{1}{x} + frac{1}{y}$ (Evaluate derivative of logarithm using $D(log{x}) = frac{1}{x}$



$frac{1}{xy} = frac{1}{x} + frac{1}{y}$ looks false to me; e.g. while $log{6}$ does equal $log{2} + log{3}$, $frac{1}{6}$ does not equal $frac{1}{2} + frac{1}{3}$.



My first guess was that the issue was related to what variable I take the derivative with respect to, but I'd like to understand this a little more formally if someone could guide me.



What's going wrong in this example?










share|cite|improve this question











$endgroup$




Given the property of the logarithm that $log{xy} = log{x} + log{y}$, how would one take the 'derivative' of this?



To be more clear,



$log{xy} = log{x} + log{y}$ (property of $log$)



$D(log{xy}) = D(log{x} + log{y})$ (i) (Take derivative on both sides)



Now, $D(log{x} + log{y}) = D(log{x}) + D(log{y})$ (ii) (Derivative of sum is sum of derivatives)



Combining (i) and (ii): $D(log{xy}) = D(log{x}) + D(log{y})$ (iii)



Implies: $frac{1}{xy} = frac{1}{x} + frac{1}{y}$ (Evaluate derivative of logarithm using $D(log{x}) = frac{1}{x}$



$frac{1}{xy} = frac{1}{x} + frac{1}{y}$ looks false to me; e.g. while $log{6}$ does equal $log{2} + log{3}$, $frac{1}{6}$ does not equal $frac{1}{2} + frac{1}{3}$.



My first guess was that the issue was related to what variable I take the derivative with respect to, but I'd like to understand this a little more formally if someone could guide me.



What's going wrong in this example?







real-analysis calculus logarithms






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edited Jan 5 at 4:33









David G. Stork

10.7k31332




10.7k31332










asked Jan 5 at 4:14









user2192320user2192320

595




595












  • $begingroup$
    Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
    $endgroup$
    – MathIsLife12
    Jan 5 at 4:19










  • $begingroup$
    (This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
    $endgroup$
    – Clement C.
    Jan 5 at 4:21












  • $begingroup$
    Thanks guys; this clears things up! I appreciate the fast responses.
    $endgroup$
    – user2192320
    Jan 5 at 4:24










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – John Doe
    Jan 5 at 5:01


















  • $begingroup$
    Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
    $endgroup$
    – MathIsLife12
    Jan 5 at 4:19










  • $begingroup$
    (This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
    $endgroup$
    – Clement C.
    Jan 5 at 4:21












  • $begingroup$
    Thanks guys; this clears things up! I appreciate the fast responses.
    $endgroup$
    – user2192320
    Jan 5 at 4:24










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – John Doe
    Jan 5 at 5:01
















$begingroup$
Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
$endgroup$
– MathIsLife12
Jan 5 at 4:19




$begingroup$
Like the comment above said- you can’t take derivatives with respect to all different variables and expect them to be equal.
$endgroup$
– MathIsLife12
Jan 5 at 4:19












$begingroup$
(This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
$endgroup$
– Clement C.
Jan 5 at 4:21






$begingroup$
(This comment was from me -- deleted when reading the fact that ayt the end of your question you acknowledged something was sketchy in the variables you differentiated with respect to. Essentially, I was saying you were "differentiating simultaneously wrt 3 variables," in different places: $xy$, $x$, $y$ -- and that's not allowed.)
$endgroup$
– Clement C.
Jan 5 at 4:21














$begingroup$
Thanks guys; this clears things up! I appreciate the fast responses.
$endgroup$
– user2192320
Jan 5 at 4:24




$begingroup$
Thanks guys; this clears things up! I appreciate the fast responses.
$endgroup$
– user2192320
Jan 5 at 4:24












$begingroup$
After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
$endgroup$
– John Doe
Jan 5 at 5:01




$begingroup$
After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
$endgroup$
– John Doe
Jan 5 at 5:01










2 Answers
2






active

oldest

votes


















22












$begingroup$

Yes, as you noted, it matters which variable you take a derivative with respect to. Since we have two variables, if we assume they are independent, then we need to use partial derivatives. If we chose $x$, then $$frac{partial log(xy)}{partial x}=frac{y}{xy}=frac1x$$and $$frac{partial}{partial x}(log x+log y)=frac1x+0=frac1x$$So the answers agree. (the same thing happens if we chose $y$ instead).



Alternatively, you could take a total derivative. $$mathrm d(log xy)=frac{partial log xy}{partial x}mathrm dx+frac{partial log xy}{partial y}mathrm dy=frac1xmathrm dx+frac1y mathrm dy$$This agrees with $$d(log x+log y)=frac1xmathrm dx+frac1y mathrm dy$$
So there are no inconsistencies.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you for the quick response; this is my favorite answer posted so far.
    $endgroup$
    – user2192320
    Jan 5 at 4:25










  • $begingroup$
    No problem, happy to help!
    $endgroup$
    – John Doe
    Jan 5 at 5:02








  • 2




    $begingroup$
    @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
    $endgroup$
    – Ruslan
    Jan 5 at 16:24










  • $begingroup$
    Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
    $endgroup$
    – user2192320
    Jan 16 at 20:10



















10












$begingroup$

The issue is that your differential operator $D$ does not behave in the way you think it does!
$$
D(log(xy))=frac{1}{xy}D(xy)=frac{1}{xy}(ydx+xdy)=frac{1}{x}dx+frac{1}{y}dy=D(log(x))+D(log(y))
$$

At no point in this computation do we actually have $frac{1}{xy}=frac{1}{x}+frac{1}{y}$ -- we are working with differentials, and not derivatives.






share|cite|improve this answer









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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    22












    $begingroup$

    Yes, as you noted, it matters which variable you take a derivative with respect to. Since we have two variables, if we assume they are independent, then we need to use partial derivatives. If we chose $x$, then $$frac{partial log(xy)}{partial x}=frac{y}{xy}=frac1x$$and $$frac{partial}{partial x}(log x+log y)=frac1x+0=frac1x$$So the answers agree. (the same thing happens if we chose $y$ instead).



    Alternatively, you could take a total derivative. $$mathrm d(log xy)=frac{partial log xy}{partial x}mathrm dx+frac{partial log xy}{partial y}mathrm dy=frac1xmathrm dx+frac1y mathrm dy$$This agrees with $$d(log x+log y)=frac1xmathrm dx+frac1y mathrm dy$$
    So there are no inconsistencies.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Thank you for the quick response; this is my favorite answer posted so far.
      $endgroup$
      – user2192320
      Jan 5 at 4:25










    • $begingroup$
      No problem, happy to help!
      $endgroup$
      – John Doe
      Jan 5 at 5:02








    • 2




      $begingroup$
      @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
      $endgroup$
      – Ruslan
      Jan 5 at 16:24










    • $begingroup$
      Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
      $endgroup$
      – user2192320
      Jan 16 at 20:10
















    22












    $begingroup$

    Yes, as you noted, it matters which variable you take a derivative with respect to. Since we have two variables, if we assume they are independent, then we need to use partial derivatives. If we chose $x$, then $$frac{partial log(xy)}{partial x}=frac{y}{xy}=frac1x$$and $$frac{partial}{partial x}(log x+log y)=frac1x+0=frac1x$$So the answers agree. (the same thing happens if we chose $y$ instead).



    Alternatively, you could take a total derivative. $$mathrm d(log xy)=frac{partial log xy}{partial x}mathrm dx+frac{partial log xy}{partial y}mathrm dy=frac1xmathrm dx+frac1y mathrm dy$$This agrees with $$d(log x+log y)=frac1xmathrm dx+frac1y mathrm dy$$
    So there are no inconsistencies.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Thank you for the quick response; this is my favorite answer posted so far.
      $endgroup$
      – user2192320
      Jan 5 at 4:25










    • $begingroup$
      No problem, happy to help!
      $endgroup$
      – John Doe
      Jan 5 at 5:02








    • 2




      $begingroup$
      @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
      $endgroup$
      – Ruslan
      Jan 5 at 16:24










    • $begingroup$
      Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
      $endgroup$
      – user2192320
      Jan 16 at 20:10














    22












    22








    22





    $begingroup$

    Yes, as you noted, it matters which variable you take a derivative with respect to. Since we have two variables, if we assume they are independent, then we need to use partial derivatives. If we chose $x$, then $$frac{partial log(xy)}{partial x}=frac{y}{xy}=frac1x$$and $$frac{partial}{partial x}(log x+log y)=frac1x+0=frac1x$$So the answers agree. (the same thing happens if we chose $y$ instead).



    Alternatively, you could take a total derivative. $$mathrm d(log xy)=frac{partial log xy}{partial x}mathrm dx+frac{partial log xy}{partial y}mathrm dy=frac1xmathrm dx+frac1y mathrm dy$$This agrees with $$d(log x+log y)=frac1xmathrm dx+frac1y mathrm dy$$
    So there are no inconsistencies.






    share|cite|improve this answer









    $endgroup$



    Yes, as you noted, it matters which variable you take a derivative with respect to. Since we have two variables, if we assume they are independent, then we need to use partial derivatives. If we chose $x$, then $$frac{partial log(xy)}{partial x}=frac{y}{xy}=frac1x$$and $$frac{partial}{partial x}(log x+log y)=frac1x+0=frac1x$$So the answers agree. (the same thing happens if we chose $y$ instead).



    Alternatively, you could take a total derivative. $$mathrm d(log xy)=frac{partial log xy}{partial x}mathrm dx+frac{partial log xy}{partial y}mathrm dy=frac1xmathrm dx+frac1y mathrm dy$$This agrees with $$d(log x+log y)=frac1xmathrm dx+frac1y mathrm dy$$
    So there are no inconsistencies.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Jan 5 at 4:21









    John DoeJohn Doe

    11.1k11238




    11.1k11238












    • $begingroup$
      Thank you for the quick response; this is my favorite answer posted so far.
      $endgroup$
      – user2192320
      Jan 5 at 4:25










    • $begingroup$
      No problem, happy to help!
      $endgroup$
      – John Doe
      Jan 5 at 5:02








    • 2




      $begingroup$
      @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
      $endgroup$
      – Ruslan
      Jan 5 at 16:24










    • $begingroup$
      Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
      $endgroup$
      – user2192320
      Jan 16 at 20:10


















    • $begingroup$
      Thank you for the quick response; this is my favorite answer posted so far.
      $endgroup$
      – user2192320
      Jan 5 at 4:25










    • $begingroup$
      No problem, happy to help!
      $endgroup$
      – John Doe
      Jan 5 at 5:02








    • 2




      $begingroup$
      @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
      $endgroup$
      – Ruslan
      Jan 5 at 16:24










    • $begingroup$
      Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
      $endgroup$
      – user2192320
      Jan 16 at 20:10
















    $begingroup$
    Thank you for the quick response; this is my favorite answer posted so far.
    $endgroup$
    – user2192320
    Jan 5 at 4:25




    $begingroup$
    Thank you for the quick response; this is my favorite answer posted so far.
    $endgroup$
    – user2192320
    Jan 5 at 4:25












    $begingroup$
    No problem, happy to help!
    $endgroup$
    – John Doe
    Jan 5 at 5:02






    $begingroup$
    No problem, happy to help!
    $endgroup$
    – John Doe
    Jan 5 at 5:02






    2




    2




    $begingroup$
    @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
    $endgroup$
    – Ruslan
    Jan 5 at 16:24




    $begingroup$
    @user2192320 you may want to accept it then by clicking the tick mark on the left of the answer.
    $endgroup$
    – Ruslan
    Jan 5 at 16:24












    $begingroup$
    Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
    $endgroup$
    – user2192320
    Jan 16 at 20:10




    $begingroup$
    Hey Ruslan, sorry for the late follow up; I accepted the response and checked the tick mark. Thanks for the guidance.
    $endgroup$
    – user2192320
    Jan 16 at 20:10











    10












    $begingroup$

    The issue is that your differential operator $D$ does not behave in the way you think it does!
    $$
    D(log(xy))=frac{1}{xy}D(xy)=frac{1}{xy}(ydx+xdy)=frac{1}{x}dx+frac{1}{y}dy=D(log(x))+D(log(y))
    $$

    At no point in this computation do we actually have $frac{1}{xy}=frac{1}{x}+frac{1}{y}$ -- we are working with differentials, and not derivatives.






    share|cite|improve this answer









    $endgroup$


















      10












      $begingroup$

      The issue is that your differential operator $D$ does not behave in the way you think it does!
      $$
      D(log(xy))=frac{1}{xy}D(xy)=frac{1}{xy}(ydx+xdy)=frac{1}{x}dx+frac{1}{y}dy=D(log(x))+D(log(y))
      $$

      At no point in this computation do we actually have $frac{1}{xy}=frac{1}{x}+frac{1}{y}$ -- we are working with differentials, and not derivatives.






      share|cite|improve this answer









      $endgroup$
















        10












        10








        10





        $begingroup$

        The issue is that your differential operator $D$ does not behave in the way you think it does!
        $$
        D(log(xy))=frac{1}{xy}D(xy)=frac{1}{xy}(ydx+xdy)=frac{1}{x}dx+frac{1}{y}dy=D(log(x))+D(log(y))
        $$

        At no point in this computation do we actually have $frac{1}{xy}=frac{1}{x}+frac{1}{y}$ -- we are working with differentials, and not derivatives.






        share|cite|improve this answer









        $endgroup$



        The issue is that your differential operator $D$ does not behave in the way you think it does!
        $$
        D(log(xy))=frac{1}{xy}D(xy)=frac{1}{xy}(ydx+xdy)=frac{1}{x}dx+frac{1}{y}dy=D(log(x))+D(log(y))
        $$

        At no point in this computation do we actually have $frac{1}{xy}=frac{1}{x}+frac{1}{y}$ -- we are working with differentials, and not derivatives.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 5 at 4:19









        ItsJustTranscendenceBroItsJustTranscendenceBro

        1712




        1712






























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