Is there a way to link equations/ expressions such that that the final form is simplified?

I wish to write the first equation to be able to use for a simple lsqcurvefit script.

The data which I will be using for the curve-fitting is stress (P) and stretch (Lamda) to determine the parameters u and N.

How can I write the equation for the stress such it is expressed in terms of the stretch without the subscript cu.

See the following. I wish to eliminate stretch_cu in favour of stretch without increasing the length of the current expression. Is this possible?

stress = @(x,stretch)(x(1)/3)*((3*x(2) - stretch_cu.^2)./(x(2) - stretch_cu.^2))*(stretch - stretch.^(-2));

Answer 1

[SALAH ALRABEEI]

It is better to simplify by hand first, for examle let lambda^2 = l

then (3N-l)/(N-l)=(2N+N-l)/(N-l)= 2N/(N-l) - 1.

The same for the l, simplify it then combine the whole equation