Aims\ To solve the problems of the directional derivative of the multivariable functions. Methods\ We can simplify multivariable function with the help of single variable function. Results\ We change the directional derivative of the multivariable functions into the left and right derivative and the progressive derivative of the singlevariable functions with the help of the definition and the property of the singlevariable functions. As a result

the difficult point in the study of the infinitesimal calculus can be solved. Conclusion\ In the process of the study of the infinitesimal calculus

we can use the simple knowledge which has been learned to simplify the more difficult contents.