SOLVE command with SYMS

Hi,

I have the Symbolic Math Toolbox. However, I can't even perform the following operation, which is copied and pasted from the Mathworks website on the usage of Symbolic Math Toolbx.

   syms x
   solve(x^2 + 4*x + 1 == 0)

The error says the following:

   ??? Input argument "x0" is undefined.

   Error in ==> solve at 56
   x=x0;

This isn't really what I'm trying to do. I have a system of non-linear equations and I get a whole different error messages, but the fact that I can't even perform a simple solving operation such as the above makes me think that there's something fundamentally wrong.

Can you please help me?

Thanks,

Eric

[Walter Roberson]

Please check that you are getting the right solve() with

which -all solve

Also, which version are you using? The ability to put in logical expressions is quite new.

[Eric Hong]

Yes, I think it is the version issue. I have 7.12.0 (R2011a).

I did have a script with the same name 'solve. m'

I removed it from the path and now it says:

>> which -all solve C:\Program Files\MATLAB\R2011a\toolbox\symbolic\symbolic\solve.m

Now I have a different error message:

   ??? 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{:});

But now the same operation without the logical equal sign now works.

Thanks,

Eric

[Walter Roberson]

Using the == in that context was not yet allowed in R2011a.

Answer 1

[Azzi Abdelmalek]

   syms x
   solve(x^2 + 4*x + 1)

[Azzi Abdelmalek]

why do you need ==

[Azzi Abdelmalek]

to solve x^2 + 4*x + 1=5*x+1

solve(x^2 + 4*x + 1-(5*x+1))

[Azzi Abdelmalek]

or you don't need to use syms variables

solve('x^2 + 4*x + 1=0')