We overview differential and integral calculus learned at high school. Our aim is to generalize these to functions of several variables. For functions with one variable, first we extend the concept of tangent (linear approximation) to the theory of Taylor (polynomial approximation). As an application, we can solve the extreme problem in detail. For functions of several valuables, derivative is called partial derivative. We extend the theory of Taylor and the extreme problem to functions with several variables. Moreover, we consider integral of functions of several variables, which is called multiple integral. By using this we can obtain volume and area of high dimensional objects.