\frac{\partial }{{\partial x_i }}f\left( {a_1 ,...,a_n } \right)\mathop = {\lim }\limits_{h \to 0} \frac{{f\left( {a_1 ,...,a_{i - 1} ,a_i + h,a_{i + 1} ,...,a_n } \right) - f\left( {a_1 ,...,a_n } \right)}}{h}