Solution for $(acdot x+b)cdot sin(x)+ccdot cos(x)=0$?












0












$begingroup$


As part of an engineering problem, I've been trying for a generic solution for $(acdot x+b)cdot sin(x)+ccdot cos(x)=0$



Here's something I tried:



$(a⋅x+b)⋅tan(x)+c=0$, when $cos(x)neq 0$
$$tan(x)=frac{-c}{a⋅x+b}$$
It sort of looking like this, when, say, $a=2, b=3, c=5$:a=2, b=3, c=5



Any chance that can be solved without using numerical methods?



EDIT: I was looking for solutions in the form of $x approx f(a,b,c) $, preferably finding solution in any range, because $a$, $b$, and $c$ can take wierd values. It's used for a program where it will not be trivial to solve problem procedurally.










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$endgroup$












  • $begingroup$
    I think not possible!
    $endgroup$
    – gimusi
    Dec 13 '18 at 15:06










  • $begingroup$
    No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
    $endgroup$
    – Winther
    Dec 13 '18 at 15:10








  • 2




    $begingroup$
    No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
    $endgroup$
    – Yves Daoust
    Dec 13 '18 at 15:11
















0












$begingroup$


As part of an engineering problem, I've been trying for a generic solution for $(acdot x+b)cdot sin(x)+ccdot cos(x)=0$



Here's something I tried:



$(a⋅x+b)⋅tan(x)+c=0$, when $cos(x)neq 0$
$$tan(x)=frac{-c}{a⋅x+b}$$
It sort of looking like this, when, say, $a=2, b=3, c=5$:a=2, b=3, c=5



Any chance that can be solved without using numerical methods?



EDIT: I was looking for solutions in the form of $x approx f(a,b,c) $, preferably finding solution in any range, because $a$, $b$, and $c$ can take wierd values. It's used for a program where it will not be trivial to solve problem procedurally.










share|cite|improve this question











$endgroup$












  • $begingroup$
    I think not possible!
    $endgroup$
    – gimusi
    Dec 13 '18 at 15:06










  • $begingroup$
    No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
    $endgroup$
    – Winther
    Dec 13 '18 at 15:10








  • 2




    $begingroup$
    No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
    $endgroup$
    – Yves Daoust
    Dec 13 '18 at 15:11














0












0








0





$begingroup$


As part of an engineering problem, I've been trying for a generic solution for $(acdot x+b)cdot sin(x)+ccdot cos(x)=0$



Here's something I tried:



$(a⋅x+b)⋅tan(x)+c=0$, when $cos(x)neq 0$
$$tan(x)=frac{-c}{a⋅x+b}$$
It sort of looking like this, when, say, $a=2, b=3, c=5$:a=2, b=3, c=5



Any chance that can be solved without using numerical methods?



EDIT: I was looking for solutions in the form of $x approx f(a,b,c) $, preferably finding solution in any range, because $a$, $b$, and $c$ can take wierd values. It's used for a program where it will not be trivial to solve problem procedurally.










share|cite|improve this question











$endgroup$




As part of an engineering problem, I've been trying for a generic solution for $(acdot x+b)cdot sin(x)+ccdot cos(x)=0$



Here's something I tried:



$(a⋅x+b)⋅tan(x)+c=0$, when $cos(x)neq 0$
$$tan(x)=frac{-c}{a⋅x+b}$$
It sort of looking like this, when, say, $a=2, b=3, c=5$:a=2, b=3, c=5



Any chance that can be solved without using numerical methods?



EDIT: I was looking for solutions in the form of $x approx f(a,b,c) $, preferably finding solution in any range, because $a$, $b$, and $c$ can take wierd values. It's used for a program where it will not be trivial to solve problem procedurally.







trigonometry






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edited Dec 13 '18 at 18:45







zyc

















asked Dec 13 '18 at 14:58









zyczyc

1114




1114












  • $begingroup$
    I think not possible!
    $endgroup$
    – gimusi
    Dec 13 '18 at 15:06










  • $begingroup$
    No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
    $endgroup$
    – Winther
    Dec 13 '18 at 15:10








  • 2




    $begingroup$
    No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
    $endgroup$
    – Yves Daoust
    Dec 13 '18 at 15:11


















  • $begingroup$
    I think not possible!
    $endgroup$
    – gimusi
    Dec 13 '18 at 15:06










  • $begingroup$
    No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
    $endgroup$
    – Winther
    Dec 13 '18 at 15:10








  • 2




    $begingroup$
    No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
    $endgroup$
    – Yves Daoust
    Dec 13 '18 at 15:11
















$begingroup$
I think not possible!
$endgroup$
– gimusi
Dec 13 '18 at 15:06




$begingroup$
I think not possible!
$endgroup$
– gimusi
Dec 13 '18 at 15:06












$begingroup$
No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
$endgroup$
– Winther
Dec 13 '18 at 15:10






$begingroup$
No. See the similar type of question Numerical solution to $x = tan (x)$ for some info on how to get numerical solutions
$endgroup$
– Winther
Dec 13 '18 at 15:10






2




2




$begingroup$
No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
$endgroup$
– Yves Daoust
Dec 13 '18 at 15:11




$begingroup$
No chance. Rewrite as $cot x=px+q$ and find the intersections between the line and the vertical asymptotes to get starting approximations.
$endgroup$
– Yves Daoust
Dec 13 '18 at 15:11










2 Answers
2






active

oldest

votes


















1












$begingroup$

There's no closed form solution for that equation, so numerical methods are required.



That said, you can come up with estimates for the solutions towards $pm infty$.
For instance, as $nrightarrow +infty$, if you write the solutions as $x_n=npi + u_n$, with $u_nin (-frac pi 2, frac pi 2)$, then
$$tan x_n = tan u_n = frac {-c}{ax_n+b}tag{1}=-frac c {anpi + a u_n + b}$$
Because the right-hand side of that equation is equivalent to $-frac c {api n}$, we must have that $tan u_nrightarrow 0$, which means that $u_nrightarrow 0$.
Plugging that information back into (1) yields
$$u_n=-frac c {api n} + v_n$$ where $v_n = o(frac 1 n)$.
And you can now plug this back, again, into (1), and find an estimate for $v_n$. And you keep doing that ad lib to find higher order estimates in the solution
$$x_n = npi -frac c {api n} + ....$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
    $endgroup$
    – zyc
    Dec 13 '18 at 16:22





















1












$begingroup$

Besides numerical solutions for particular parameter values, you could try series solutions. Thus when $c$ is small compared to $a$ and $b$, the solution near $x=0$ is



$$ -frac{b}{a} + cotleft(frac{b}{a}right) frac{c}{a} + cotleft(frac{b}{a}right) csc^2 left(frac{b}{a}right) frac{c^2}{a^2}
+ left(cot^3left(frac{b}{a}right) + cotleft(frac{b}{a}right) sec^4 left(frac{b}{a}right)right) frac{c^3}{a^3} + ldots $$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
    $endgroup$
    – zyc
    Dec 13 '18 at 16:03










  • $begingroup$
    Exactly what I need! Is there a tutorial on how to derive that?
    $endgroup$
    – zyc
    Dec 13 '18 at 16:04











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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









1












$begingroup$

There's no closed form solution for that equation, so numerical methods are required.



That said, you can come up with estimates for the solutions towards $pm infty$.
For instance, as $nrightarrow +infty$, if you write the solutions as $x_n=npi + u_n$, with $u_nin (-frac pi 2, frac pi 2)$, then
$$tan x_n = tan u_n = frac {-c}{ax_n+b}tag{1}=-frac c {anpi + a u_n + b}$$
Because the right-hand side of that equation is equivalent to $-frac c {api n}$, we must have that $tan u_nrightarrow 0$, which means that $u_nrightarrow 0$.
Plugging that information back into (1) yields
$$u_n=-frac c {api n} + v_n$$ where $v_n = o(frac 1 n)$.
And you can now plug this back, again, into (1), and find an estimate for $v_n$. And you keep doing that ad lib to find higher order estimates in the solution
$$x_n = npi -frac c {api n} + ....$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
    $endgroup$
    – zyc
    Dec 13 '18 at 16:22


















1












$begingroup$

There's no closed form solution for that equation, so numerical methods are required.



That said, you can come up with estimates for the solutions towards $pm infty$.
For instance, as $nrightarrow +infty$, if you write the solutions as $x_n=npi + u_n$, with $u_nin (-frac pi 2, frac pi 2)$, then
$$tan x_n = tan u_n = frac {-c}{ax_n+b}tag{1}=-frac c {anpi + a u_n + b}$$
Because the right-hand side of that equation is equivalent to $-frac c {api n}$, we must have that $tan u_nrightarrow 0$, which means that $u_nrightarrow 0$.
Plugging that information back into (1) yields
$$u_n=-frac c {api n} + v_n$$ where $v_n = o(frac 1 n)$.
And you can now plug this back, again, into (1), and find an estimate for $v_n$. And you keep doing that ad lib to find higher order estimates in the solution
$$x_n = npi -frac c {api n} + ....$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
    $endgroup$
    – zyc
    Dec 13 '18 at 16:22
















1












1








1





$begingroup$

There's no closed form solution for that equation, so numerical methods are required.



That said, you can come up with estimates for the solutions towards $pm infty$.
For instance, as $nrightarrow +infty$, if you write the solutions as $x_n=npi + u_n$, with $u_nin (-frac pi 2, frac pi 2)$, then
$$tan x_n = tan u_n = frac {-c}{ax_n+b}tag{1}=-frac c {anpi + a u_n + b}$$
Because the right-hand side of that equation is equivalent to $-frac c {api n}$, we must have that $tan u_nrightarrow 0$, which means that $u_nrightarrow 0$.
Plugging that information back into (1) yields
$$u_n=-frac c {api n} + v_n$$ where $v_n = o(frac 1 n)$.
And you can now plug this back, again, into (1), and find an estimate for $v_n$. And you keep doing that ad lib to find higher order estimates in the solution
$$x_n = npi -frac c {api n} + ....$$






share|cite|improve this answer









$endgroup$



There's no closed form solution for that equation, so numerical methods are required.



That said, you can come up with estimates for the solutions towards $pm infty$.
For instance, as $nrightarrow +infty$, if you write the solutions as $x_n=npi + u_n$, with $u_nin (-frac pi 2, frac pi 2)$, then
$$tan x_n = tan u_n = frac {-c}{ax_n+b}tag{1}=-frac c {anpi + a u_n + b}$$
Because the right-hand side of that equation is equivalent to $-frac c {api n}$, we must have that $tan u_nrightarrow 0$, which means that $u_nrightarrow 0$.
Plugging that information back into (1) yields
$$u_n=-frac c {api n} + v_n$$ where $v_n = o(frac 1 n)$.
And you can now plug this back, again, into (1), and find an estimate for $v_n$. And you keep doing that ad lib to find higher order estimates in the solution
$$x_n = npi -frac c {api n} + ....$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 13 '18 at 15:30









Stefan LafonStefan Lafon

2,24019




2,24019












  • $begingroup$
    I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
    $endgroup$
    – zyc
    Dec 13 '18 at 16:22




















  • $begingroup$
    I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
    $endgroup$
    – zyc
    Dec 13 '18 at 16:22


















$begingroup$
I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
$endgroup$
– zyc
Dec 13 '18 at 16:22






$begingroup$
I'll try that to see if I can rewrite it as $x_n = f(a,b,c)$. I'm not a mathematician, so I'm not really sure.
$endgroup$
– zyc
Dec 13 '18 at 16:22













1












$begingroup$

Besides numerical solutions for particular parameter values, you could try series solutions. Thus when $c$ is small compared to $a$ and $b$, the solution near $x=0$ is



$$ -frac{b}{a} + cotleft(frac{b}{a}right) frac{c}{a} + cotleft(frac{b}{a}right) csc^2 left(frac{b}{a}right) frac{c^2}{a^2}
+ left(cot^3left(frac{b}{a}right) + cotleft(frac{b}{a}right) sec^4 left(frac{b}{a}right)right) frac{c^3}{a^3} + ldots $$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
    $endgroup$
    – zyc
    Dec 13 '18 at 16:03










  • $begingroup$
    Exactly what I need! Is there a tutorial on how to derive that?
    $endgroup$
    – zyc
    Dec 13 '18 at 16:04
















1












$begingroup$

Besides numerical solutions for particular parameter values, you could try series solutions. Thus when $c$ is small compared to $a$ and $b$, the solution near $x=0$ is



$$ -frac{b}{a} + cotleft(frac{b}{a}right) frac{c}{a} + cotleft(frac{b}{a}right) csc^2 left(frac{b}{a}right) frac{c^2}{a^2}
+ left(cot^3left(frac{b}{a}right) + cotleft(frac{b}{a}right) sec^4 left(frac{b}{a}right)right) frac{c^3}{a^3} + ldots $$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    $$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
    $endgroup$
    – zyc
    Dec 13 '18 at 16:03










  • $begingroup$
    Exactly what I need! Is there a tutorial on how to derive that?
    $endgroup$
    – zyc
    Dec 13 '18 at 16:04














1












1








1





$begingroup$

Besides numerical solutions for particular parameter values, you could try series solutions. Thus when $c$ is small compared to $a$ and $b$, the solution near $x=0$ is



$$ -frac{b}{a} + cotleft(frac{b}{a}right) frac{c}{a} + cotleft(frac{b}{a}right) csc^2 left(frac{b}{a}right) frac{c^2}{a^2}
+ left(cot^3left(frac{b}{a}right) + cotleft(frac{b}{a}right) sec^4 left(frac{b}{a}right)right) frac{c^3}{a^3} + ldots $$






share|cite|improve this answer









$endgroup$



Besides numerical solutions for particular parameter values, you could try series solutions. Thus when $c$ is small compared to $a$ and $b$, the solution near $x=0$ is



$$ -frac{b}{a} + cotleft(frac{b}{a}right) frac{c}{a} + cotleft(frac{b}{a}right) csc^2 left(frac{b}{a}right) frac{c^2}{a^2}
+ left(cot^3left(frac{b}{a}right) + cotleft(frac{b}{a}right) sec^4 left(frac{b}{a}right)right) frac{c^3}{a^3} + ldots $$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 13 '18 at 15:31









Robert IsraelRobert Israel

325k23214468




325k23214468












  • $begingroup$
    $$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
    $endgroup$
    – zyc
    Dec 13 '18 at 16:03










  • $begingroup$
    Exactly what I need! Is there a tutorial on how to derive that?
    $endgroup$
    – zyc
    Dec 13 '18 at 16:04


















  • $begingroup$
    $$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
    $endgroup$
    – zyc
    Dec 13 '18 at 16:03










  • $begingroup$
    Exactly what I need! Is there a tutorial on how to derive that?
    $endgroup$
    – zyc
    Dec 13 '18 at 16:04
















$begingroup$
$$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
$endgroup$
– zyc
Dec 13 '18 at 16:03




$begingroup$
$$left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot sec^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=1right )+left ( sum_{i=1}^{infty }left ( cot^{i}frac{b}{a}+cotfrac{b}{a}cdot csc^{2left ( i-1 right )}frac{b}{a} right )cdot frac{c^{i-1}}{a^{i-1}}mid imod 2=0right )$$
$endgroup$
– zyc
Dec 13 '18 at 16:03












$begingroup$
Exactly what I need! Is there a tutorial on how to derive that?
$endgroup$
– zyc
Dec 13 '18 at 16:04




$begingroup$
Exactly what I need! Is there a tutorial on how to derive that?
$endgroup$
– zyc
Dec 13 '18 at 16:04


















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