main, t(domain), 'REPLACE_WITH_DASH_DASHb', 'LineWidth', 2.5);
legend([h1, h2], {'f(x)', 'f_{quad}(x)'}, 'Location', 'NorthWest');
plot(Xk, f(Xk), '.k', 'MarkerSize', 25);
plot([Xk, Xk], [ymin, f(Xk)], ':k');
plot(maximum, t(maximum),'.k', 'MarkerSize', 25);
plot([maximum,  maximum], [ymin, t(maximum)], ':k');
% x^(k)
annotation(gcf,'textbox'        , [0.65 0 0.09946 0.11] , ...
               'String'         , {'x_{k}'}             , ...
               'FontSize'       , 18                    , ...
      