f[x_]:=E^(x/3);
nodes := {0, 0.1, 0.2, 0.3};
n:=Length[nodes] -1;
nPlusOneDerivative [x_]:=(E^(x/3))/3^(n+1);  
w[x_]:=Product[(x-nodes[[i]]), {i, 1, n+1}];
minDerivative = Minimize[{Abs[nPlusOneDerivative[x]] ,nodes[[1]]≤x≤nodes[[n+1]]},{x}][[1]];
minWx = Minimize[{Abs[w[x]], nodes[[1]]≤x≤nodes[[n+1]]},{x}][[1]];
maxDerivative = Maximize[{Abs[nPlusOneDerivative[x]] ,nodes[[1]]≤x≤nodes[[n+1]]},{x}][[1]];
maxWx = Maximize[{Abs[w[x]], nodes[[1]]≤x≤nodes[[n+1]]},{x}][[1]];
minError = minWx*minDerivative/(n+1)! (* груби оценки са - не се достигат *)
maxError = maxWx*maxDerivative/(n+1)! (* груби оценки са - не се достигат *)
Plot[{Abs[w[x]]*minDerivative/(n+1)!, Abs[w[x]]*maxDerivative/(n+1)!},{x, nodes[[1]], nodes[[n+1]]}]