Skip to main content

Solve doesn't give an answer with system of equations


I have to solve the following system of equations:


Solve[{k*Sech[d]*Cos[b]/Sqrt[A^2 + B^2] == 1, 
k*Sech[d]*Sin[b]/Sqrt[A^2 + B^2] == 0, A*k*Tanh[d]/(A^2 + B^2)+c1 == 1,

B*k*Tanh[d]/(A^2 + B^2)+c2 == 0, k*Sech[k + d]*Cos[a + b]/Sqrt[A^2 + B^2] == 1,
k*Sech[k + d]*Sin[a + b]/Sqrt[A^2 + B^2] == 0.1,
A*k*Tanh[k + d]/(A^2 + B^2)+c1 == 1.1,
B*k*Tanh[k + d]/(A^2 + B^2)+c2 == 0}, {k, d, a, b, A, B, c1, c2}]

but Solve will just compute forever and give no result. I've tried also with Reduce and NSolve, but with no luck. I can use approximated answers, so I've tried to use the Taylor series of the second order of the functions Sech Tanh Cos and Sin, but still Solve couldn't give me an answer.


I there something else I could try?


Thank you.



Answer



Notice that most of your transcendal functions are neatly separated into pairs of equations.



This will get you most of the way to your solution


sys={k*Sech[d]*Cos[b]/Sqrt[A^2+B^2]==1, k*Sech[d]*Sin[b]/Sqrt[A^2+B^2]==0, 
A*k*Tanh[d]/(A^2+B^2)+c1==1, B*k*Tanh[d]/(A^2+B^2)+c2==0,
k*Sech[k+d]*Cos[a+b]/Sqrt[A^2+B^2]==1, k*Sech[k+d]*Sin[a+b]/Sqrt[A^2+B^2]==1/10,
A*k*Tanh[k+d]/(A^2+B^2)+c1==11/10, B*k*Tanh[k+d]/(A^2+B^2)+c2==0};
sol=Eliminate[sys, {Sech[d], Tanh[d], Sech[k+d], Tanh[k+d]}]

which instantly tells you


(* B==0 && c2==0 && Cos[a+b]==10 Sin[a+b] && Sin[b]==0 && A!=0 *)


Follow that with


sys /. ToRules[sol] /. True -> Sequence[]

which instantly tells you


(* {(k Cos[b] Sech[d])/Sqrt[A^2] == 1,
(10 k Sech[d+k] Sin[a+b])/Sqrt[A^2]==1,
(k Sech[d+k] Sin[a+b])/Sqrt[A^2]==1/10,
c1+(k Tanh[d])/A==1,
c1+(k Tanh[d+k])/A==11/10}*)


Notice one of those equations is obviously redundant and can be eliminated. Then notice c1 can then be eliminated giving even one fewer equation. Then notice that information from the first step is telling you that b is a multiple of Pi and Cos[b] is either +1 or -1.


If you make use of all this information then you might be able to use Reduce to get to the final solution.


Comments

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...