I would not use the Symbolic Math Toolbox for iterative numerical calculations. It is inefficient for that purpose.
I would use fzero and anonymous function implementations of ‘l’ and the expression you want to solve for in the loop (that I call ‘fcn’ here):
c1= 0.5176; c2= 116; c3= 0.4; c4= 5; c5= 21; c6= 0.0068;
l = @(beta,lambda2) (1/(lambda2+0.008*beta))-(0.035/((beta^3)+1)); % Create Anonymous Function
lambda_m = [7.943485689; 7.876168014; 7.809981728; 7.744898547; 7.680891121; ... 7.876168014; 7.943485689];
cp_m = [0.3862077656; 0.3764718995; 0.3670605433; 0.3579602882; 0.3491583852; ... 0.3764718995; 0.3862077656];
beta_m=zeros(67,1); i=1; while i<=67 cp2=cp_m(i,:); lambda2=lambda_m(i,:); fcn = @(beta) (c1*(c2*l(beta,lambda2)-c3*beta-c4)*exp(-c5*l(beta,lambda2))+c6) - cp2; R(i) = fzero(fcn, 1) i=i+1 end
Experiment to get the result you want.