math.random([n [,m]])
文档原版：
this function is an interface to the simple pseudo-random generator function rand provided by ANSI C. (No guarantees can be given for its statistical properties.)
When called without arguments, returns a pseudo-random real number in the range [0,1).
When called with a number m, math.random returns a pseudo-random integer in the range [1, m].
When called with two numbers m and n, math.random returns a pseudo-random integer in the range [m, n].
翻译：
用法：1.无参调用，产生[0, 1)之间的浮点随机数。
2.一个参数n，产生[1, n]之间的整数。
3.两个参数，产生[n, m]之间的整数