Get coeffients of symbolic expression

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Mech Princess
Mech Princess 2012-7-21
I have a set of equations (derv_alpha) that I have arrived at by differentiating an expression and setting it to zero.
N=3; alphavec = sym('alpha',[1 N]); %N can take any value
derv_alpha = [ 0, 2*alphavec(3) - 2*alphavec(2) + 2, 2*alphavec(2) - 4*alphavec(3)];
Now I need to solve these simultaneous equations and come up with answers for alpha2, alpha3...alphaN. i.e. alphavec(2), alphavec(3)...alphavec(N).
I am trying to put this in matrix form A*x=b and solve for x. Is it possible? If so, HOW CAN I GET THE COEFFICIENTS OF EACH VARIABLE?
The solution will have to be irrespective of the position of the variable in the expression.
I did a quick search. collect() doesnt work. i dont think sym2poly can be used
OR, is there a better way to do it when working with symbols?
I am doing my first program using symbols this week and appreciate any help. Thanks again.

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回答（1 个）

Walter Roberson
Walter Roberson 2012-7-21
Generally,
solve(derv_alpha, alphavec)
However, derv_alpha(1) is the constant 0 and not an expression in the alpha variables, so the set of three equations in derv_alpha, in three variables, is really only two equations in three variables, which you aren't going to be able to solve for. This is going to be a problem even if you express as A*x=b
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Mech Princess
Mech Princess 2012-7-21
Thanks. It's an output from another line of code which is the partial derivative of an expression. The spaces are what MATLAB gives and frankly, I was wondering about it myself. Thanks
Mech Princess
Mech Princess 2012-7-21
does anyone know how to do this? Thanks

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