How to solve a power/polynomial equation












0












$begingroup$


Is there a method to solve such equations : $AB^{x} + C x = D$ ? with $A$, $B$, $C$ and $D$ real parameters and $xin mathbb{R}$ and $B>0$?










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    what is $x$? real?
    $endgroup$
    – José Alejandro Aburto Araneda
    Dec 29 '18 at 16:58






  • 1




    $begingroup$
    Yes indeed, I corrected the question thank you.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 16:59










  • $begingroup$
    Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
    $endgroup$
    – Servaes
    Dec 29 '18 at 17:35










  • $begingroup$
    Yes B is assumed to B positive.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:36
















0












$begingroup$


Is there a method to solve such equations : $AB^{x} + C x = D$ ? with $A$, $B$, $C$ and $D$ real parameters and $xin mathbb{R}$ and $B>0$?










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    what is $x$? real?
    $endgroup$
    – José Alejandro Aburto Araneda
    Dec 29 '18 at 16:58






  • 1




    $begingroup$
    Yes indeed, I corrected the question thank you.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 16:59










  • $begingroup$
    Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
    $endgroup$
    – Servaes
    Dec 29 '18 at 17:35










  • $begingroup$
    Yes B is assumed to B positive.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:36














0












0








0





$begingroup$


Is there a method to solve such equations : $AB^{x} + C x = D$ ? with $A$, $B$, $C$ and $D$ real parameters and $xin mathbb{R}$ and $B>0$?










share|cite|improve this question











$endgroup$




Is there a method to solve such equations : $AB^{x} + C x = D$ ? with $A$, $B$, $C$ and $D$ real parameters and $xin mathbb{R}$ and $B>0$?







real-analysis algebra-precalculus exponential-function






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 29 '18 at 23:49









Servaes

30.9k342101




30.9k342101










asked Dec 29 '18 at 16:57









AlexC75AlexC75

216




216








  • 2




    $begingroup$
    what is $x$? real?
    $endgroup$
    – José Alejandro Aburto Araneda
    Dec 29 '18 at 16:58






  • 1




    $begingroup$
    Yes indeed, I corrected the question thank you.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 16:59










  • $begingroup$
    Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
    $endgroup$
    – Servaes
    Dec 29 '18 at 17:35










  • $begingroup$
    Yes B is assumed to B positive.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:36














  • 2




    $begingroup$
    what is $x$? real?
    $endgroup$
    – José Alejandro Aburto Araneda
    Dec 29 '18 at 16:58






  • 1




    $begingroup$
    Yes indeed, I corrected the question thank you.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 16:59










  • $begingroup$
    Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
    $endgroup$
    – Servaes
    Dec 29 '18 at 17:35










  • $begingroup$
    Yes B is assumed to B positive.
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:36








2




2




$begingroup$
what is $x$? real?
$endgroup$
– José Alejandro Aburto Araneda
Dec 29 '18 at 16:58




$begingroup$
what is $x$? real?
$endgroup$
– José Alejandro Aburto Araneda
Dec 29 '18 at 16:58




1




1




$begingroup$
Yes indeed, I corrected the question thank you.
$endgroup$
– AlexC75
Dec 29 '18 at 16:59




$begingroup$
Yes indeed, I corrected the question thank you.
$endgroup$
– AlexC75
Dec 29 '18 at 16:59












$begingroup$
Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
$endgroup$
– Servaes
Dec 29 '18 at 17:35




$begingroup$
Is $B$ assumed to be positive? If not then $B^x$ becomes problematic...
$endgroup$
– Servaes
Dec 29 '18 at 17:35












$begingroup$
Yes B is assumed to B positive.
$endgroup$
– AlexC75
Dec 29 '18 at 17:36




$begingroup$
Yes B is assumed to B positive.
$endgroup$
– AlexC75
Dec 29 '18 at 17:36










1 Answer
1






active

oldest

votes


















1












$begingroup$

First, three degenerate cases;




  1. If $C=0$ and $B=1$ then the equation reduces to $A=D$, and there is nothing to solve.

  2. If $C=0$ and $Bneq1$ then $AB^x=D$ and hence $x=frac{ln{frac{D}{A}}}{ln{B}}$.

  3. If $Cneq0$ and $B=1$ then $A+Cx=D$ and hence $x=frac{D-A}{C}$.


If $Cneq0$ and $Bneq1$ then setting $a:=frac{A}{C}$ and $d:=frac{D}{C}$ yields
$$aB^x=d-x.$$
Setting $y:=d-x$ and $v:=B^{-1}$ and $u:=aB^d$ then yields
$$y=aB^{d-y}=aB^dB^{-y}=uv^y=ue^{yln{v}},$$
where $e$ denotes Euler's constant. Finally setting $z:=-yln{v}$ and rearranging yields the equation
$$ze^z=-uln{v},$$
which has no 'elementary' closed form solution in general. The solution to this is precisely what the Lambert W function denotes. That is, by definition we have $z=W(-uln{v})$. Substituting everything back yields
$$-left(frac{D}{C}-xright)ln{B^{-1}}=Wleft(-frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right),$$
and isolating $x$ shows that
$$x=frac{D}{C}-frac{1}{ln{B}}Wleft(frac{A}{C}B^{frac{D}{C}}ln{B}right).$$
Finding exact values of the Lambert W function in terms of elementary functions is in general not possible, though of course there are practical algorithms for numerical approximations.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:59












  • $begingroup$
    See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:03












  • $begingroup$
    Thanks a lot. Do you know if this list is exhaustive ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 18:11












  • $begingroup$
    I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:21






  • 1




    $begingroup$
    Should it not be $ze^z = - u ln(v) $ ?
    $endgroup$
    – AlexC75
    Dec 30 '18 at 17:28














Your Answer








StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3056036%2fhow-to-solve-a-power-polynomial-equation%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1












$begingroup$

First, three degenerate cases;




  1. If $C=0$ and $B=1$ then the equation reduces to $A=D$, and there is nothing to solve.

  2. If $C=0$ and $Bneq1$ then $AB^x=D$ and hence $x=frac{ln{frac{D}{A}}}{ln{B}}$.

  3. If $Cneq0$ and $B=1$ then $A+Cx=D$ and hence $x=frac{D-A}{C}$.


If $Cneq0$ and $Bneq1$ then setting $a:=frac{A}{C}$ and $d:=frac{D}{C}$ yields
$$aB^x=d-x.$$
Setting $y:=d-x$ and $v:=B^{-1}$ and $u:=aB^d$ then yields
$$y=aB^{d-y}=aB^dB^{-y}=uv^y=ue^{yln{v}},$$
where $e$ denotes Euler's constant. Finally setting $z:=-yln{v}$ and rearranging yields the equation
$$ze^z=-uln{v},$$
which has no 'elementary' closed form solution in general. The solution to this is precisely what the Lambert W function denotes. That is, by definition we have $z=W(-uln{v})$. Substituting everything back yields
$$-left(frac{D}{C}-xright)ln{B^{-1}}=Wleft(-frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right),$$
and isolating $x$ shows that
$$x=frac{D}{C}-frac{1}{ln{B}}Wleft(frac{A}{C}B^{frac{D}{C}}ln{B}right).$$
Finding exact values of the Lambert W function in terms of elementary functions is in general not possible, though of course there are practical algorithms for numerical approximations.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:59












  • $begingroup$
    See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:03












  • $begingroup$
    Thanks a lot. Do you know if this list is exhaustive ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 18:11












  • $begingroup$
    I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:21






  • 1




    $begingroup$
    Should it not be $ze^z = - u ln(v) $ ?
    $endgroup$
    – AlexC75
    Dec 30 '18 at 17:28


















1












$begingroup$

First, three degenerate cases;




  1. If $C=0$ and $B=1$ then the equation reduces to $A=D$, and there is nothing to solve.

  2. If $C=0$ and $Bneq1$ then $AB^x=D$ and hence $x=frac{ln{frac{D}{A}}}{ln{B}}$.

  3. If $Cneq0$ and $B=1$ then $A+Cx=D$ and hence $x=frac{D-A}{C}$.


If $Cneq0$ and $Bneq1$ then setting $a:=frac{A}{C}$ and $d:=frac{D}{C}$ yields
$$aB^x=d-x.$$
Setting $y:=d-x$ and $v:=B^{-1}$ and $u:=aB^d$ then yields
$$y=aB^{d-y}=aB^dB^{-y}=uv^y=ue^{yln{v}},$$
where $e$ denotes Euler's constant. Finally setting $z:=-yln{v}$ and rearranging yields the equation
$$ze^z=-uln{v},$$
which has no 'elementary' closed form solution in general. The solution to this is precisely what the Lambert W function denotes. That is, by definition we have $z=W(-uln{v})$. Substituting everything back yields
$$-left(frac{D}{C}-xright)ln{B^{-1}}=Wleft(-frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right),$$
and isolating $x$ shows that
$$x=frac{D}{C}-frac{1}{ln{B}}Wleft(frac{A}{C}B^{frac{D}{C}}ln{B}right).$$
Finding exact values of the Lambert W function in terms of elementary functions is in general not possible, though of course there are practical algorithms for numerical approximations.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:59












  • $begingroup$
    See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:03












  • $begingroup$
    Thanks a lot. Do you know if this list is exhaustive ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 18:11












  • $begingroup$
    I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:21






  • 1




    $begingroup$
    Should it not be $ze^z = - u ln(v) $ ?
    $endgroup$
    – AlexC75
    Dec 30 '18 at 17:28
















1












1








1





$begingroup$

First, three degenerate cases;




  1. If $C=0$ and $B=1$ then the equation reduces to $A=D$, and there is nothing to solve.

  2. If $C=0$ and $Bneq1$ then $AB^x=D$ and hence $x=frac{ln{frac{D}{A}}}{ln{B}}$.

  3. If $Cneq0$ and $B=1$ then $A+Cx=D$ and hence $x=frac{D-A}{C}$.


If $Cneq0$ and $Bneq1$ then setting $a:=frac{A}{C}$ and $d:=frac{D}{C}$ yields
$$aB^x=d-x.$$
Setting $y:=d-x$ and $v:=B^{-1}$ and $u:=aB^d$ then yields
$$y=aB^{d-y}=aB^dB^{-y}=uv^y=ue^{yln{v}},$$
where $e$ denotes Euler's constant. Finally setting $z:=-yln{v}$ and rearranging yields the equation
$$ze^z=-uln{v},$$
which has no 'elementary' closed form solution in general. The solution to this is precisely what the Lambert W function denotes. That is, by definition we have $z=W(-uln{v})$. Substituting everything back yields
$$-left(frac{D}{C}-xright)ln{B^{-1}}=Wleft(-frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right),$$
and isolating $x$ shows that
$$x=frac{D}{C}-frac{1}{ln{B}}Wleft(frac{A}{C}B^{frac{D}{C}}ln{B}right).$$
Finding exact values of the Lambert W function in terms of elementary functions is in general not possible, though of course there are practical algorithms for numerical approximations.






share|cite|improve this answer











$endgroup$



First, three degenerate cases;




  1. If $C=0$ and $B=1$ then the equation reduces to $A=D$, and there is nothing to solve.

  2. If $C=0$ and $Bneq1$ then $AB^x=D$ and hence $x=frac{ln{frac{D}{A}}}{ln{B}}$.

  3. If $Cneq0$ and $B=1$ then $A+Cx=D$ and hence $x=frac{D-A}{C}$.


If $Cneq0$ and $Bneq1$ then setting $a:=frac{A}{C}$ and $d:=frac{D}{C}$ yields
$$aB^x=d-x.$$
Setting $y:=d-x$ and $v:=B^{-1}$ and $u:=aB^d$ then yields
$$y=aB^{d-y}=aB^dB^{-y}=uv^y=ue^{yln{v}},$$
where $e$ denotes Euler's constant. Finally setting $z:=-yln{v}$ and rearranging yields the equation
$$ze^z=-uln{v},$$
which has no 'elementary' closed form solution in general. The solution to this is precisely what the Lambert W function denotes. That is, by definition we have $z=W(-uln{v})$. Substituting everything back yields
$$-left(frac{D}{C}-xright)ln{B^{-1}}=Wleft(-frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right),$$
and isolating $x$ shows that
$$x=frac{D}{C}-frac{1}{ln{B}}Wleft(frac{A}{C}B^{frac{D}{C}}ln{B}right).$$
Finding exact values of the Lambert W function in terms of elementary functions is in general not possible, though of course there are practical algorithms for numerical approximations.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 1 at 21:09

























answered Dec 29 '18 at 17:48









ServaesServaes

30.9k342101




30.9k342101












  • $begingroup$
    Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:59












  • $begingroup$
    See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:03












  • $begingroup$
    Thanks a lot. Do you know if this list is exhaustive ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 18:11












  • $begingroup$
    I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:21






  • 1




    $begingroup$
    Should it not be $ze^z = - u ln(v) $ ?
    $endgroup$
    – AlexC75
    Dec 30 '18 at 17:28




















  • $begingroup$
    Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 17:59












  • $begingroup$
    See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:03












  • $begingroup$
    Thanks a lot. Do you know if this list is exhaustive ?
    $endgroup$
    – AlexC75
    Dec 29 '18 at 18:11












  • $begingroup$
    I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
    $endgroup$
    – Servaes
    Dec 29 '18 at 18:21






  • 1




    $begingroup$
    Should it not be $ze^z = - u ln(v) $ ?
    $endgroup$
    – AlexC75
    Dec 30 '18 at 17:28


















$begingroup$
Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
$endgroup$
– AlexC75
Dec 29 '18 at 17:59






$begingroup$
Thank you very much for this, very interesting stuff. What would be the conditions on A,B,C and D for which $Wleft(frac{A}{C}B^{frac{D}{C}}ln{B^{-1}}right)$ could be expressed in terms of elementary function ?
$endgroup$
– AlexC75
Dec 29 '18 at 17:59














$begingroup$
See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
$endgroup$
– Servaes
Dec 29 '18 at 18:03






$begingroup$
See the wikipedia section on special values for a few known values of $W(z)$, which you can then translate to conditions on $A$, $B$, $C$ and $D$. One condition that particularly jumps out is $A=0$, and then obviously $x=frac{D}{C}$, but otherwise the expression seems too convoluted to say anything nice about. But I could be wrong.
$endgroup$
– Servaes
Dec 29 '18 at 18:03














$begingroup$
Thanks a lot. Do you know if this list is exhaustive ?
$endgroup$
– AlexC75
Dec 29 '18 at 18:11






$begingroup$
Thanks a lot. Do you know if this list is exhaustive ?
$endgroup$
– AlexC75
Dec 29 '18 at 18:11














$begingroup$
I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
$endgroup$
– Servaes
Dec 29 '18 at 18:21




$begingroup$
I have no doubt there are much more extensive lists, though I don't know where to find them. If there are particular values you are interested in, perhaps wolfram alpha can help you.
$endgroup$
– Servaes
Dec 29 '18 at 18:21




1




1




$begingroup$
Should it not be $ze^z = - u ln(v) $ ?
$endgroup$
– AlexC75
Dec 30 '18 at 17:28






$begingroup$
Should it not be $ze^z = - u ln(v) $ ?
$endgroup$
– AlexC75
Dec 30 '18 at 17:28




















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematics Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3056036%2fhow-to-solve-a-power-polynomial-equation%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







Popular posts from this blog

Le Mesnil-Réaume

Ida-Boy-Ed-Garten

web3.py web3.isConnected() returns false always