A transportation company stipulates that the freight rate for goods per ton kilometers is: within kilometers, yuan per kilometer, and for the excess kilometer, yuan per kilometer. Find the functional relationship between freight price and mileage .

　　Solution: According to the meaning of the question, the following functional relationships 　can be listed :

,

Here, the functional relationship between freight rate and mileage is represented by a piecewise function , and the domain of this function is the interval .