using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static double func(double x)
        {
            return Math.Tan(x) - (Math.Pow(Math.Tan(x), 3)) / 3 + (Math.Pow(Math.Tan(x), 5)) / 5 - 0.1 / 0.3;
        }

        static double sfunc(double x)
        {
            return (3*Math.Cos(4*x)+5)/Math.Pow(Math.Cos(x), 6)/8;
        }

        static void Main(string[] args)
        {
            double a = 0, b = 0.8;
            double eps = 0.0001;

            double x1 = 0, x2 = x1 - func(x1) / sfunc(x1);

            while (Math.Abs(x2 - x1) > (Math.Sqrt(eps) / 10))
            {
                x1 = x2;
                x2 = x1 - func(x1) / sfunc(x1);
            }

            double answer = x2;

            Console.WriteLine("{0:f4}",answer);
            Console.ReadKey();
        }
    }
}