求平方根可了解一下 Newton's method，求三角函数可了解一下 CORDIC、Taylor series（Maclaurin series）、Padé approximant 和 range reduction [1]。
计算器的各种计算原理可参考 [2]。
[1] Payne, Mary H., and Robert N. Hanek. "Radian reduction for trigonometric functions."ACM SIGNUM Newsletter18.1 (1983): 19-24.
[2] Rising, Gerald R.Inside your calculator: from simple programs to significant insights. John Wiley & Sons, 2007.
