122 g

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

namespace _122_zadacha_g_
{
    class Program
    {
        static void Main(string[] args)
        {
            double x, eps;
            do
            {
                Console.Write("Enter x: ");
                x = double.Parse(Console.ReadLine());
            } while (Math.Abs(x) > 1);

            do
            {
                Console.Write("Enter eps: ");
                eps = double.Parse(Console.ReadLine());
            } while (eps <= 0 || eps >= 0.01);


            double y, a, i;
            y = x;
            a = x;
            i = 1;
            double u = (2 * i) + 1;

            do
            {
                a = Math.Pow(-1, i) * Math.Pow(x, ((2 * i) + 1));
                u = (2 * i) + 1;
                y += a / u;
                i++;
            } while (Math.Abs(a / i) >= eps);

            Console.WriteLine("y = {0}; ArcTg({1}) = {2}", y, x, Math.Atan(x));
        }
    }
}