From help-request at octave dot org Mon Feb  7 21:54:38 2005
Subject: Re: 2 linear eq
From: Geordie McBain <geordie dot mcbain at aeromech dot usyd dot edu dot au>
To: shih lin <shlinlin at 37 dot com>
Cc: help at octave dot org
Date: Tue, 8 Feb 2005 13:44:29 -0500

On Mon, Feb 07, 2005 at 07:27:49PM -0800, shih lin wrote:
> to solve  8x + y =-29
> 8x - y = -19
> 
> I do
> 
> >> function z=f(x)
>   z=[8*x(1)+x(2)=-29, 8*x(1)-x(2)=-19];
>    endfunction
> >> fsolve("f",[0,0])
> error: invalid lvalue function called in expression
> error: evaluating assignment expression near line 2, column 17
> error: evaluating assignment expression near line 2, column 4
> error: called from `f'
> error: fsolve: evaluation of user-supplied function failed
> >> 
> I am in GNU Octave, version 2.1.42 (i686-pc-cygwin), if anyone can point how to 
> upgrade I am highly appreciate(I get it just follow download website's 
> instruction)
> 
> eric

No, the function passed to fsolve shouldn't contain the equations, it
should return the residuals.  Here you'd need

  function z = f(x)
    z = [8,1;8,-1]*x + [29;19];
  endfunction

This works O.K.:

  octave> fsolve ("f", [0;0])
  ans =

    -3.0000
    -5.0000

However, since your equations are linear, you don't really need fsolve
(it's only required for nonlinear equations).  You can solve linear
systems like yours by:

octave> [8,1;8,-1] \ [-29;-19]
ans =

  -3
  -5

Geordie McBain
www.aeromech.usyd.edu.au/~mcbain




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