Symbolic equation with numeric variable

Question

Rashmil Dahanayake 2013-9-23

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Is it possible to use a numeric variable(in this case "alfa") in a symbolic equation.

I'm trying to solve the symbolic q. I need the flexibility to change the value of alfa. However if i use the term "alfa" when defining the eqn, marlab fails to solve the equation.

If I manually type the value of alfa inside the equation, the programs runs without a problem.

f=50;
w=2*pi*f;
R=303.47/50;
alfa=72;
syms q;
eqn = 'sin(72*pi/180) == cos(q) * exp(tan(q) * (pi/6 - 72*pi/180 + q))';
% This is the format I want to use,but gives errors
%----------------------------------------------------------------------
%eqn = 'sin(alfa*pi/180) == cos(q) * exp(tan(q) * (pi/6 - alfa*pi/180 + q))';  
%----------------------------------------------------------------------
y= double(solve(eqn));
C=1/(w*R*tan(y)) /10^(-6)  % Capacitor Value

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Azzi Abdelmalek 2013-9-23

What is the error message?

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Answer 1

Walter Roberson 2013-9-23

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编辑：Walter Roberson 2013-9-23

在 MATLAB Online 中打开

eqn = subs('sin(alfa*pi/180) = cos(q) * exp(tan(q) * (pi/6 - alfa*pi/180 + q))', 'alfa', alfa);

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Answer 2

Rashmil Dahanayake 2013-9-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/87929-symbolic-equation-with-numeric-variable#answer_97613

编辑：Azzi Abdelmalek 2013-9-24

在 MATLAB Online 中打开

Subs method worked.

Alternatively you can define the eqn without quotation (') marks.

Therefore following code is also correct.

eqn = sin(alfa*pi/180) == cos(q) * exp(tan(q) * (pi/6 - alfa*pi/180 + q));
y= double(solve(eqn));

I'm wondering for symbolic solver is there any benefit passing arguments as a string.(ie using quotation marks)

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Walter Roberson 2013-9-24

When you pass as a string, you need to use MuPAD syntax. But in turn that means you can access MuPAD syntax and operators that do not have a direct MATLAB interface.

Or so the theory goes; I have not had my hands on a MuPAD version to experiment with.

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