go Atanh()，反双曲正切
Atanh()方法语法
func Atanh(x float64) float64
go源码对Atanh()方法的介绍：
Atanh returns the inverse hyperbolic tangent of x.
Special cases are:
Atanh(1) = +Inf
Atanh(±0) = ±0
Atanh(-1) = -Inf
Atanh(x) = NaN if x < -1 or x > 1
Atanh(NaN) = NaNgolang模块math的Atanh()方法，求取flaot64类型参数x的反双曲正切值，除此之外，Atanh(1) = +Inf；Atanh(±0) = ±0；Atanh(-1) = -Inf；Atanh(x) = NaN if x < -1 or x > 1;Atanh(NaN) = NaN。
math.Atanh()方法实例代码
func main() {
atanh_v := math.Atanh(0.2)
fmt.Println(atanh_v)
}运行go文件，得到输出：
0.2027325540540822