F(h_1,h_2)\approx F(h_1^*,h_2^*)+\left.\dfrac{\partial F}{\partial h_1}\right|_{h_1^*,h_2^*}(h_1-h_1^*)+\left.\dfrac{\partial F}{\partial h_2}\right|_{h_1^*,h_2^*}(h_2-h_2^*)