Using several variables with fsolve

Question

Nate 2012-4-16

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https://ww2.mathworks.cn/matlabcentral/answers/35678-using-several-variables-with-fsolve

I have a 2x1 force matrix that I'd like to solve for 0. The problem right now is that it is completely made up of 6 variables and I'd like matlab to return solutions for 2 of the variables.

fsolve doesn't seem to be able to deal with this many variables.

For the matrix:

Ft=[3*Yc*k*exp(i*omega*t) - (23*L*k*theta*exp(i*omega*t))/20 - (5*L*Yc*m*omega^2*exp(i*omega*t))/2 ;
(443*L^2*k*theta*exp(i*omega*t))/400 - (23*L*Yc*k*exp(i*omega*t))/20 - (99*L^3*m*omega^2*theta*exp(i*omega*t))/160]
solve(Ft==0,Yc,theta)

I've even tried using:

solve(Ft(1,:)==0,Ft(2,:)==0,Yc,theta)

as well as just even:

solve(Ft(1,:)==0,Yc)

I would like to return solutions for Yc and theta.

The error I'm receiving for all methods is the following:

??? Error using ==> char
Conversion to char from logical is not possible.
Error in ==> solve>getEqns at 165
   vc = char(v);
Error in ==> solve at 67
[eqns,vars] = getEqns(varargin{:});

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Answer 1

Walter Roberson 2012-4-16

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We are going to need to see your call to solve() and the equations you are passing in.

You would get the above error if you are passing in a logical value, which could (for example) happen if you use "==" in your equations.

The general solution for the matrix you show is Yc = 0, theta = 0. There are additional solutions such as Yc = 0, L = 0, theta = anything.

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Nate 2012-4-17

added information, thank you for assisting.

Walter Roberson 2012-4-17

Leave out the "== 0" . That is converting your inputs to boolean. Solving for the expressions being equal to zero is the default.

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Answer 2

Sargondjani 2012-4-17

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在 MATLAB Online 中打开

yes, walther is right (of course), you need to keep the syntax for fsolve:

YOur objective is: Ft=0, so pass as objective just 'Ft'.

BUT you have to write it as a function. First you have to specifiy the parameters, and then the function:

par1=...
par2=...
myfunction=@(var1,var2,...,var_last)[...;...];

on the ... in the [] you can write your orginal thingy, with the parameters and variables.

then you solve with fsolve:

fsolve(myfunction,[var1_0,var2_0,...var_last_0])

this is the format you should be using. BUT you have to understand that fsolve will only give 1 solution! in your case, there infinitely many solutions, so you need to work on that first (as the other poster was saying)