The symbolic expression on the right side of the equals sign:
L*[(bet(i)*vtet(i))/(tet(i)^2);-(alf(i)*vtet(i))/(tet(i)^2);(stet(i))/tet(i)];
still contains a symbolic variable. When you try assigning into the double array plcl (which is double because it was preallocated using zeros) MATLAB can't evaluate the symbolic expression to get a number. It's as though I asked you the numeric value of sin(z) without telling you what z is. Is it -1, 1, or something inbetween?
Your two main choices:
- Substitute values into the symbolic expression using subs so the result is a number, containing no symbolic variables.
- Preallocate plcl to be a symbolic array using sym(zeros(3, 1)).