Error returned with solve function

Question

Uku 2012-7-7

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/43010-error-returned-with-solve-function

Hello!

I'm a bit puzzled when trying to solve a simple equation, taken as an example from the Matlab documentation:

syms a b c x
solve(a*x^2 + b*x + c == 0)

This returns:

??? Error using ==> char Conversion to char from logical is not possible.

Error in ==> solve>getEqns at 189 vc = char(v);

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

Quite the opposite to what the example displays.

How can I solve this? Uku

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Answer 1

Walter Roberson 2012-7-7

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https://ww2.mathworks.cn/matlabcentral/answers/43010-error-returned-with-solve-function#answer_52867

You are using an older version of MATLAB that evaluates the truth of the comparison expression (creating a logical) before it calls solve()

Alternately, you might have assignments to a, b, c, and x before the solve() call, and if those assignments are all numeric values then the "symbolness" of the variables would be over-written, leaving a*x^2 + b*x + c as being a numeric variable, whose numeric value would be compared to 0 and that result passed to solve()