\frac{\partial }{{\partial x_j }}f_i \left( c \right) = \mathop {\lim }\limits_{h \to 0} \frac{{f_i \left( {c_1 ,...,c_{j - 1} ,c_j + h,c_{j + 1} ,...,c_n } \right) - f_i \left( {c_1 ,...,c_n } \right)}}{h}