Calculating multiple parameters for a logarithmic function

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I have the following function -



$$y=ln(Ax^D+B+Cx^E)$$



These are the coordinates:

$$(1, 1)$$$$(2, 0.84)$$$$(4, 1.5)$$$$(31, 4.1)$$$$(44, 5)$$



How can you solve this equation and what is the correct method for finding the $A,B,C,D$ and $E$ parameters?



EDIT



It seems that the last coordinate is causing a problem.

Anyway what's the correct way to find the equation without the last coordinate and as close as possible?







share|cite|improve this question

















  • 2




    desmos.com/calculator/ragn6n1qqr
    – Mason
    Jul 15 at 8:38










  • I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
    – Drxxd
    Jul 15 at 8:41






  • 1




    Where did you get this problem from?
    – TheSimpliFire
    Jul 15 at 8:47






  • 1




    @Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
    – Davide Morgante
    Jul 15 at 9:04






  • 1




    @Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
    – Davide Morgante
    Jul 15 at 13:57














up vote
2
down vote

favorite












I have the following function -



$$y=ln(Ax^D+B+Cx^E)$$



These are the coordinates:

$$(1, 1)$$$$(2, 0.84)$$$$(4, 1.5)$$$$(31, 4.1)$$$$(44, 5)$$



How can you solve this equation and what is the correct method for finding the $A,B,C,D$ and $E$ parameters?



EDIT



It seems that the last coordinate is causing a problem.

Anyway what's the correct way to find the equation without the last coordinate and as close as possible?







share|cite|improve this question

















  • 2




    desmos.com/calculator/ragn6n1qqr
    – Mason
    Jul 15 at 8:38










  • I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
    – Drxxd
    Jul 15 at 8:41






  • 1




    Where did you get this problem from?
    – TheSimpliFire
    Jul 15 at 8:47






  • 1




    @Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
    – Davide Morgante
    Jul 15 at 9:04






  • 1




    @Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
    – Davide Morgante
    Jul 15 at 13:57












up vote
2
down vote

favorite









up vote
2
down vote

favorite











I have the following function -



$$y=ln(Ax^D+B+Cx^E)$$



These are the coordinates:

$$(1, 1)$$$$(2, 0.84)$$$$(4, 1.5)$$$$(31, 4.1)$$$$(44, 5)$$



How can you solve this equation and what is the correct method for finding the $A,B,C,D$ and $E$ parameters?



EDIT



It seems that the last coordinate is causing a problem.

Anyway what's the correct way to find the equation without the last coordinate and as close as possible?







share|cite|improve this question













I have the following function -



$$y=ln(Ax^D+B+Cx^E)$$



These are the coordinates:

$$(1, 1)$$$$(2, 0.84)$$$$(4, 1.5)$$$$(31, 4.1)$$$$(44, 5)$$



How can you solve this equation and what is the correct method for finding the $A,B,C,D$ and $E$ parameters?



EDIT



It seems that the last coordinate is causing a problem.

Anyway what's the correct way to find the equation without the last coordinate and as close as possible?









share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited Jul 15 at 9:07









TheSimpliFire

9,69261951




9,69261951









asked Jul 15 at 8:34









Drxxd

1305




1305







  • 2




    desmos.com/calculator/ragn6n1qqr
    – Mason
    Jul 15 at 8:38










  • I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
    – Drxxd
    Jul 15 at 8:41






  • 1




    Where did you get this problem from?
    – TheSimpliFire
    Jul 15 at 8:47






  • 1




    @Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
    – Davide Morgante
    Jul 15 at 9:04






  • 1




    @Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
    – Davide Morgante
    Jul 15 at 13:57












  • 2




    desmos.com/calculator/ragn6n1qqr
    – Mason
    Jul 15 at 8:38










  • I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
    – Drxxd
    Jul 15 at 8:41






  • 1




    Where did you get this problem from?
    – TheSimpliFire
    Jul 15 at 8:47






  • 1




    @Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
    – Davide Morgante
    Jul 15 at 9:04






  • 1




    @Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
    – Davide Morgante
    Jul 15 at 13:57







2




2




desmos.com/calculator/ragn6n1qqr
– Mason
Jul 15 at 8:38




desmos.com/calculator/ragn6n1qqr
– Mason
Jul 15 at 8:38












I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
– Drxxd
Jul 15 at 8:41




I've also tried to play with desmos but I couldn't get the exact function, so the question is what's the correct way to solve this
– Drxxd
Jul 15 at 8:41




1




1




Where did you get this problem from?
– TheSimpliFire
Jul 15 at 8:47




Where did you get this problem from?
– TheSimpliFire
Jul 15 at 8:47




1




1




@Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
– Davide Morgante
Jul 15 at 9:04




@Mason fitting the points in semi-log scale gives $R^2 = 1$ even with the fifth point desmos.com/calculator/vm1eh3qb28 sorry for the link before, I just realised that I gave your link instead of mine
– Davide Morgante
Jul 15 at 9:04




1




1




@Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
– Davide Morgante
Jul 15 at 13:57




@Mason ahah don't worry! It wasn't my brightest moment, got wrong too many things! My line of reasoning wanted to be the same as the answer of@YvesDaoust
– Davide Morgante
Jul 15 at 13:57










2 Answers
2






active

oldest

votes

















up vote
1
down vote



accepted










Too long for a comment, but may be of some use:



I have omitted the last set of coordinates as the OP has requested.



Substitute $(1,1)$: $$A+B+C=etag1$$



Substitute $(2,0.84)$: $$Acdot2^D+Ccdot2^E+B=e^0.84tag2$$



Substitute $(4,1.5)$: $$Acdot2^2D+Ccdot2^2E+B=e^4tag3$$



Substitute $(31,4.1)$: $$Acdot31^D+Ccdot31^E+B=e^4.1tag4$$



Do $(3)-(2)$: $$Acdot2^D(2^D-1)+Ccdot2^E(2^E-1)=e^4-e^0.84tag5$$



Do $(2)-(1)$: $$Acdot(2^D-1)+Ccdot(2^E-1)=e^0.84-etag6$$



Simultaneous equation so solve for $A$ and $C$ in terms of $D$ and $E$.



Multiply $(7)$ by $2^E$: $$Acdot2^E(2^D-1)+Ccdot2^E(2^E-1)=2^E(e^4-e^0.84)tag7$$



Do $(8)-(6)$: $$A(2^E-2^D)(2^D-1)=(2^E-1)(e^4-e^0.84)$$ so $$A=frac(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag8$$



Substitute $(9)$ into $(7)$: $$C=frac(2^E-2^D)(e^0.84-e)-(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag9$$



Substitute $(9)$ and $(10)$ into $(1)$: $$B=e-frace^0.84-e2^D-1tag10$$



Now $A$ and $C$ are in terms of $D$ and $E$, but $B$ is only in terms of $D$.






share|cite|improve this answer























  • If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
    – TheSimpliFire
    Jul 15 at 9:10










  • That's great for this moment.. Thanks!
    – Drxxd
    Jul 15 at 9:18






  • 1




    No, $4^D=(2^2)^D=2^2Dneq2^D+1$
    – TheSimpliFire
    Jul 16 at 10:43






  • 1




    the concept is still the same whether it is 1.5 or 4
    – TheSimpliFire
    Jul 16 at 10:43







  • 1




    Are you sure? Try $D=5$...
    – TheSimpliFire
    Jul 16 at 13:18

















up vote
0
down vote













The model



$$Ax^D+B+Cx^E=e^y$$ is partly linear and partly nonlinear and has no closed-form solution. You can make it slightly more tractable numerically by plugging the coordinates of the five points to obtain five equations, and eliminate the coefficients $A,B,C$ by forming linear combinations (for instance, you can solve the three first equations by Cramer and plug in the remaining two).



In the end, you will obtain as simplified system of two equations in the two unknowns $D$ and $E$ (though these equations involve ten exponentials). This can be solved by Newton in a less costly way than the original system, and as there are only two independent variables left, graphical observation is possible.






share|cite|improve this answer























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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    1
    down vote



    accepted










    Too long for a comment, but may be of some use:



    I have omitted the last set of coordinates as the OP has requested.



    Substitute $(1,1)$: $$A+B+C=etag1$$



    Substitute $(2,0.84)$: $$Acdot2^D+Ccdot2^E+B=e^0.84tag2$$



    Substitute $(4,1.5)$: $$Acdot2^2D+Ccdot2^2E+B=e^4tag3$$



    Substitute $(31,4.1)$: $$Acdot31^D+Ccdot31^E+B=e^4.1tag4$$



    Do $(3)-(2)$: $$Acdot2^D(2^D-1)+Ccdot2^E(2^E-1)=e^4-e^0.84tag5$$



    Do $(2)-(1)$: $$Acdot(2^D-1)+Ccdot(2^E-1)=e^0.84-etag6$$



    Simultaneous equation so solve for $A$ and $C$ in terms of $D$ and $E$.



    Multiply $(7)$ by $2^E$: $$Acdot2^E(2^D-1)+Ccdot2^E(2^E-1)=2^E(e^4-e^0.84)tag7$$



    Do $(8)-(6)$: $$A(2^E-2^D)(2^D-1)=(2^E-1)(e^4-e^0.84)$$ so $$A=frac(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag8$$



    Substitute $(9)$ into $(7)$: $$C=frac(2^E-2^D)(e^0.84-e)-(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag9$$



    Substitute $(9)$ and $(10)$ into $(1)$: $$B=e-frace^0.84-e2^D-1tag10$$



    Now $A$ and $C$ are in terms of $D$ and $E$, but $B$ is only in terms of $D$.






    share|cite|improve this answer























    • If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
      – TheSimpliFire
      Jul 15 at 9:10










    • That's great for this moment.. Thanks!
      – Drxxd
      Jul 15 at 9:18






    • 1




      No, $4^D=(2^2)^D=2^2Dneq2^D+1$
      – TheSimpliFire
      Jul 16 at 10:43






    • 1




      the concept is still the same whether it is 1.5 or 4
      – TheSimpliFire
      Jul 16 at 10:43







    • 1




      Are you sure? Try $D=5$...
      – TheSimpliFire
      Jul 16 at 13:18














    up vote
    1
    down vote



    accepted










    Too long for a comment, but may be of some use:



    I have omitted the last set of coordinates as the OP has requested.



    Substitute $(1,1)$: $$A+B+C=etag1$$



    Substitute $(2,0.84)$: $$Acdot2^D+Ccdot2^E+B=e^0.84tag2$$



    Substitute $(4,1.5)$: $$Acdot2^2D+Ccdot2^2E+B=e^4tag3$$



    Substitute $(31,4.1)$: $$Acdot31^D+Ccdot31^E+B=e^4.1tag4$$



    Do $(3)-(2)$: $$Acdot2^D(2^D-1)+Ccdot2^E(2^E-1)=e^4-e^0.84tag5$$



    Do $(2)-(1)$: $$Acdot(2^D-1)+Ccdot(2^E-1)=e^0.84-etag6$$



    Simultaneous equation so solve for $A$ and $C$ in terms of $D$ and $E$.



    Multiply $(7)$ by $2^E$: $$Acdot2^E(2^D-1)+Ccdot2^E(2^E-1)=2^E(e^4-e^0.84)tag7$$



    Do $(8)-(6)$: $$A(2^E-2^D)(2^D-1)=(2^E-1)(e^4-e^0.84)$$ so $$A=frac(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag8$$



    Substitute $(9)$ into $(7)$: $$C=frac(2^E-2^D)(e^0.84-e)-(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag9$$



    Substitute $(9)$ and $(10)$ into $(1)$: $$B=e-frace^0.84-e2^D-1tag10$$



    Now $A$ and $C$ are in terms of $D$ and $E$, but $B$ is only in terms of $D$.






    share|cite|improve this answer























    • If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
      – TheSimpliFire
      Jul 15 at 9:10










    • That's great for this moment.. Thanks!
      – Drxxd
      Jul 15 at 9:18






    • 1




      No, $4^D=(2^2)^D=2^2Dneq2^D+1$
      – TheSimpliFire
      Jul 16 at 10:43






    • 1




      the concept is still the same whether it is 1.5 or 4
      – TheSimpliFire
      Jul 16 at 10:43







    • 1




      Are you sure? Try $D=5$...
      – TheSimpliFire
      Jul 16 at 13:18












    up vote
    1
    down vote



    accepted







    up vote
    1
    down vote



    accepted






    Too long for a comment, but may be of some use:



    I have omitted the last set of coordinates as the OP has requested.



    Substitute $(1,1)$: $$A+B+C=etag1$$



    Substitute $(2,0.84)$: $$Acdot2^D+Ccdot2^E+B=e^0.84tag2$$



    Substitute $(4,1.5)$: $$Acdot2^2D+Ccdot2^2E+B=e^4tag3$$



    Substitute $(31,4.1)$: $$Acdot31^D+Ccdot31^E+B=e^4.1tag4$$



    Do $(3)-(2)$: $$Acdot2^D(2^D-1)+Ccdot2^E(2^E-1)=e^4-e^0.84tag5$$



    Do $(2)-(1)$: $$Acdot(2^D-1)+Ccdot(2^E-1)=e^0.84-etag6$$



    Simultaneous equation so solve for $A$ and $C$ in terms of $D$ and $E$.



    Multiply $(7)$ by $2^E$: $$Acdot2^E(2^D-1)+Ccdot2^E(2^E-1)=2^E(e^4-e^0.84)tag7$$



    Do $(8)-(6)$: $$A(2^E-2^D)(2^D-1)=(2^E-1)(e^4-e^0.84)$$ so $$A=frac(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag8$$



    Substitute $(9)$ into $(7)$: $$C=frac(2^E-2^D)(e^0.84-e)-(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag9$$



    Substitute $(9)$ and $(10)$ into $(1)$: $$B=e-frace^0.84-e2^D-1tag10$$



    Now $A$ and $C$ are in terms of $D$ and $E$, but $B$ is only in terms of $D$.






    share|cite|improve this answer















    Too long for a comment, but may be of some use:



    I have omitted the last set of coordinates as the OP has requested.



    Substitute $(1,1)$: $$A+B+C=etag1$$



    Substitute $(2,0.84)$: $$Acdot2^D+Ccdot2^E+B=e^0.84tag2$$



    Substitute $(4,1.5)$: $$Acdot2^2D+Ccdot2^2E+B=e^4tag3$$



    Substitute $(31,4.1)$: $$Acdot31^D+Ccdot31^E+B=e^4.1tag4$$



    Do $(3)-(2)$: $$Acdot2^D(2^D-1)+Ccdot2^E(2^E-1)=e^4-e^0.84tag5$$



    Do $(2)-(1)$: $$Acdot(2^D-1)+Ccdot(2^E-1)=e^0.84-etag6$$



    Simultaneous equation so solve for $A$ and $C$ in terms of $D$ and $E$.



    Multiply $(7)$ by $2^E$: $$Acdot2^E(2^D-1)+Ccdot2^E(2^E-1)=2^E(e^4-e^0.84)tag7$$



    Do $(8)-(6)$: $$A(2^E-2^D)(2^D-1)=(2^E-1)(e^4-e^0.84)$$ so $$A=frac(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag8$$



    Substitute $(9)$ into $(7)$: $$C=frac(2^E-2^D)(e^0.84-e)-(2^E-1)(e^4-e^0.84)(2^E-2^D)(2^D-1)tag9$$



    Substitute $(9)$ and $(10)$ into $(1)$: $$B=e-frace^0.84-e2^D-1tag10$$



    Now $A$ and $C$ are in terms of $D$ and $E$, but $B$ is only in terms of $D$.







    share|cite|improve this answer















    share|cite|improve this answer



    share|cite|improve this answer








    edited Jul 15 at 9:04


























    answered Jul 15 at 8:59









    TheSimpliFire

    9,69261951




    9,69261951











    • If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
      – TheSimpliFire
      Jul 15 at 9:10










    • That's great for this moment.. Thanks!
      – Drxxd
      Jul 15 at 9:18






    • 1




      No, $4^D=(2^2)^D=2^2Dneq2^D+1$
      – TheSimpliFire
      Jul 16 at 10:43






    • 1




      the concept is still the same whether it is 1.5 or 4
      – TheSimpliFire
      Jul 16 at 10:43







    • 1




      Are you sure? Try $D=5$...
      – TheSimpliFire
      Jul 16 at 13:18
















    • If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
      – TheSimpliFire
      Jul 15 at 9:10










    • That's great for this moment.. Thanks!
      – Drxxd
      Jul 15 at 9:18






    • 1




      No, $4^D=(2^2)^D=2^2Dneq2^D+1$
      – TheSimpliFire
      Jul 16 at 10:43






    • 1




      the concept is still the same whether it is 1.5 or 4
      – TheSimpliFire
      Jul 16 at 10:43







    • 1




      Are you sure? Try $D=5$...
      – TheSimpliFire
      Jul 16 at 13:18















    If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
    – TheSimpliFire
    Jul 15 at 9:10




    If we had another equation then the expressions for $A,B,C$ could be substituted into the remaining two equations and could be solved simultaneously.
    – TheSimpliFire
    Jul 15 at 9:10












    That's great for this moment.. Thanks!
    – Drxxd
    Jul 15 at 9:18




    That's great for this moment.. Thanks!
    – Drxxd
    Jul 15 at 9:18




    1




    1




    No, $4^D=(2^2)^D=2^2Dneq2^D+1$
    – TheSimpliFire
    Jul 16 at 10:43




    No, $4^D=(2^2)^D=2^2Dneq2^D+1$
    – TheSimpliFire
    Jul 16 at 10:43




    1




    1




    the concept is still the same whether it is 1.5 or 4
    – TheSimpliFire
    Jul 16 at 10:43





    the concept is still the same whether it is 1.5 or 4
    – TheSimpliFire
    Jul 16 at 10:43





    1




    1




    Are you sure? Try $D=5$...
    – TheSimpliFire
    Jul 16 at 13:18




    Are you sure? Try $D=5$...
    – TheSimpliFire
    Jul 16 at 13:18










    up vote
    0
    down vote













    The model



    $$Ax^D+B+Cx^E=e^y$$ is partly linear and partly nonlinear and has no closed-form solution. You can make it slightly more tractable numerically by plugging the coordinates of the five points to obtain five equations, and eliminate the coefficients $A,B,C$ by forming linear combinations (for instance, you can solve the three first equations by Cramer and plug in the remaining two).



    In the end, you will obtain as simplified system of two equations in the two unknowns $D$ and $E$ (though these equations involve ten exponentials). This can be solved by Newton in a less costly way than the original system, and as there are only two independent variables left, graphical observation is possible.






    share|cite|improve this answer



























      up vote
      0
      down vote













      The model



      $$Ax^D+B+Cx^E=e^y$$ is partly linear and partly nonlinear and has no closed-form solution. You can make it slightly more tractable numerically by plugging the coordinates of the five points to obtain five equations, and eliminate the coefficients $A,B,C$ by forming linear combinations (for instance, you can solve the three first equations by Cramer and plug in the remaining two).



      In the end, you will obtain as simplified system of two equations in the two unknowns $D$ and $E$ (though these equations involve ten exponentials). This can be solved by Newton in a less costly way than the original system, and as there are only two independent variables left, graphical observation is possible.






      share|cite|improve this answer

























        up vote
        0
        down vote










        up vote
        0
        down vote









        The model



        $$Ax^D+B+Cx^E=e^y$$ is partly linear and partly nonlinear and has no closed-form solution. You can make it slightly more tractable numerically by plugging the coordinates of the five points to obtain five equations, and eliminate the coefficients $A,B,C$ by forming linear combinations (for instance, you can solve the three first equations by Cramer and plug in the remaining two).



        In the end, you will obtain as simplified system of two equations in the two unknowns $D$ and $E$ (though these equations involve ten exponentials). This can be solved by Newton in a less costly way than the original system, and as there are only two independent variables left, graphical observation is possible.






        share|cite|improve this answer















        The model



        $$Ax^D+B+Cx^E=e^y$$ is partly linear and partly nonlinear and has no closed-form solution. You can make it slightly more tractable numerically by plugging the coordinates of the five points to obtain five equations, and eliminate the coefficients $A,B,C$ by forming linear combinations (for instance, you can solve the three first equations by Cramer and plug in the remaining two).



        In the end, you will obtain as simplified system of two equations in the two unknowns $D$ and $E$ (though these equations involve ten exponentials). This can be solved by Newton in a less costly way than the original system, and as there are only two independent variables left, graphical observation is possible.







        share|cite|improve this answer















        share|cite|improve this answer



        share|cite|improve this answer








        edited Jul 15 at 9:47


























        answered Jul 15 at 9:38









        Yves Daoust

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