多线程计算pi的值，可以在程序开头更改线程数和N的值-Multi-threaded computing the value of pi, you can change the number of threads and the value of N at the beginning of the program

利用定积分的定义编写多线程程序实现计算pi的程序，尽量让精度提高。(We use the definition of definite integral to compiling multithreaded program to realize the program of calculating pi, and try to improve the accuracy.)