利用函数和递归做一个数列的加减运算，数列的加减运算有一定的规律，可利用函数与计算出结果-The constant 　can be calculated by the infinite series:



　　　　　　　　　= 4- 4/3+ 4/5- 4/7+ 4/9- 4/11+....



　　Write a C program that uses a do/while loop to calculate 　using the series. The program should ask the user how many terms in the series should be used. Thus if the user enters ‘3’, then the program should calculate 　as being 4- 4/3+ 4/5.