NewtonMethod

function Xs = NewtonRoot(Fun, FunDer, Xest, Err, imax)

    for i = 1:imax
        Xi = Xest -feval(Fun, Xest)/feval(FunDer,Xest);

        if abs((Xi - Xest)/Xest) < Err
            Xs = Xi;
            break
        end
        Xest = Xi;
    end

    if i == imax
        fprintf('Solution was not obtained in %i iteration.\n', imax)
        Xs = ('No answer');
    end
end