使用查表法来实现基本的数学函数：开平方，sin，cos和acos。 主要针对于嵌入式开发，对运算速度有要求的情况下。
Use look-up table to realized mathmatical function.
Related function: sqrt(), sin(), cos() and acos()
When working on embedded development, you usually asked for more efficient computing, instead of mathmatical function provided by cmath.h.
So I choosed the look-up table to do so.