public void Test() {
    Console.WriteLine(MyNthRoot(3.1415926));
}
/// <summary>
/// 自己写的任意数开平方
/// </summary> 
public static double MyNthRoot(double num, double precision = double.Epsilon) {
    double x = num;
    while (true) {
        double xNew = (x / 2) + num / (2 * x); //1/2+3/2 
        if (Math.Abs(xNew - x) < precision) {
            return xNew;
        }
        x = xNew;
    }
}
/// <summary>
/// AI给的任意数开任意次方
/// </summary>
public static double NthRoot(double num, int ciFang, double precision = 1e-12) {
    double x = 1.0;
    while (true) {
        double xNew = ((ciFang - 1) * x + num / Math.Pow(x, ciFang - 1)) / ciFang;
        if (Math.Abs(xNew - x) < precision)
            return xNew;
        x = xNew;
    }
}