MATLAB: Fzero: Not Enough Input Arguments

fminuncOptimization Toolbox

EDIT: I have edited this section of my question as advised by Ammer Hamza.
Problem Statement:
To minimise an underconstrained linear symbolic system equations in the form of:
a1*X1i + a2*X2i + a3*X3i + a4*X4i + a5*X5i = Yi
(where i=1 to 100)
I have 5 coefficients (X1 to X5), which I have as inputs from a spreadsheet, 5 unknown variables (a1 to a5), and 100 equations.
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Hi, I have referred to the other threads on the same error message but I am still unable to solve my problem. I have an underconstrained linear symbolic equation generated from other variables that I would like to minimise. Below is a simplified example of the script I have:
EDITED after comments from Alan Weiss:
a=sym('a',[2,1]);
a1=a(1);a2=a(2);
eqn_a(1)=a(1)+a(2)-3;
eqn_a(2)=2*a(1)+a(2)-4;
eqn_a(3)=a(1)-a(2)+1;
fun=matlabFunction(eqn_a); 
x0=[1 10];
fzero(fun,x0)
This is the error message I get:
   Error using fzero (line 241)
  FZERO cannot continue because user-supplied function_handle ==> @(a1,a2)[a1+a2-3.0,a1.*2.0+a2-4.0,a1-a2+1.0] failed with the error
  below.
   Not enough input arguments.
Additionally, is this a suitable function or the best function to solve this problem? I have 5 coefficients, 5 variables and 100 equations.
Thank you very much.

Best Answer

Since your number of equations (100) is greater than the number of variables (5), the best you can expect is a least square solution, i.e. formulate this as a least square optimization problem as you rightly mentioned in your question. Fortunately solving such system for unconstrained cases require just one line in MATLAB. You don't need to use any function such as fminunc().
X = [x1 x2 x3 x4 x5]; % values from spreadsheet, it will be a [100 x 5] matrix
Y = y; % value of |y| from the spreadsheet.
coefficients = X\Y; % that's it
coefficients will contain the value of a1 to a5 in order.
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