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Solving four equations with four unknowns in MATLAB

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Meysam 2014-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/164388-solving-four-equations-with-four-unknowns-in-matlab

评论： Meysam 2014-11-26

采纳的回答： John D'Errico

Hello every one. I need to find the coefficients R1-R4 in these four equations. Appreciate your comments and helps.

399.87=(34.78R1)*((8.52E-6)^R2)*(0.2025^R3)*(0.33^R4)

399.87=(52.3R1)*((1.9E-5)^R2)*(0.2025^R3)*(0.33^R4))

936.72=(57.5R1)*((1.9E-5)^R2)*(0.3009^R3)*(0.33^R4))

1129=(53R1)*((1.9E-5)^R2)*(0.335^R3)*(0.5^R4))

I tried this code but it didnt help:

equ1='((34.78R1)*((8.52E-6)^R2)*(0.2025^R3)*(0.33^R4))-399.87'

equ2='((52.3R1)*((1.9E-5)^R2)*(0.2025^R3)*(0.33^R4))-399.87'

equ3='((57.5R1)*((1.9E-5)^R2)*(0.3009^R3)*(0.33^R4))-936.72'

equ4='((53R1)*((1.9E-5)^R2)*(0.335^R3)*(0.5^R4))-1129'

sol=solve(equ1,equ2,equ3,equ4)

Am I using a wrong code or I didn't execute it in the correct way?

Thanks in advance.

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John D'Errico 2014-11-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/164388-solving-four-equations-with-four-unknowns-in-matlab#answer_160303

When you ask a question, it help that you add sufficient information for someone to know your problem without a guess. What did it do that you felt was wrong? If an error message was generated, then show the complete message.

I might point out that fragments like 34.78R1 are not valid syntax in MATLAB. Multiplication employs the * operator.

Beyond that, it seems clear that your equations form a linear system once they are logged. Since you have 4 equations in 4 unknowns, a linear system will yield an exact answer. So backslash would trivially suffice.

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Meysam 2014-11-26

Thanks for the information and I'm sorry for not providing complete information. Here is the error that I received:

??? Error using ==> solve>getEqns at 182 ' (34.78R1)*((8.52E-6)^R2)*(0.2025^R3)*(0.33^R4))-399.87 ' is not a valid expression or equation.

Error in ==> solve at 67 [eqns,vars] = getEqns(varargin{:});

Error in ==> meysam at 8 sol=solve(equ1,equ2,equ3,equ4)

I guess I failed at inserting the equations in the correct format. I try to correct them.

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