1.5
using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter area of the circle");
            double r = double.Parse(Console.ReadLine());
            Console.WriteLine("Enter area of the square");
            double s = double.Parse(Console.ReadLine());

            double radius = Math.Sqrt(r / Math.PI);
            double sqsize = Math.Sqrt(s);
            double sqdiag = Math.Sqrt(2) * sqsize;

            if (sqdiag <= radius * 2) {
                Console.WriteLine("Square fits into the circle");
            } else {
                Console.WriteLine("Square does not fit into the circle");
            }
        }
    }
}


2.6
using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter number of points and then one point per line");
            int n = int.Parse(Console.ReadLine());
            for (int i = 0; i < n; ++i) {
                var input = Array.ConvertAll(Console.ReadLine().Split(), double.Parse);
                double x = input[0], y = input[1];
                if (x < 0 || x > Math.PI || y < 0 || y > Math.Sin(x)) {
                    Console.WriteLine("Point is not in the area");
                } else {
                    Console.WriteLine("Point is in the area");
                }
            }
        }
    }
}


3.11
using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            const int nExams = 4;
            Console.WriteLine("Mark the end of input with '-1'");
            int nGoodStudents = 0;
            int nBadStudents = 0;
            int sumGoodMarks = 0;
            while (true) {
                var input = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
                if (input[0] == -1) {
                    break;
                }
                int a = input[0], b = input[1], c = input[2], d = input[3];
                if (a == 2 || b == 2 || c == 2 || d == 2) {
                    ++nBadStudents;
                } else {
                    ++nGoodStudents;
                    sumGoodMarks += a + b + c + d;
                }
            }
            Console.WriteLine("There are {0} bad students", nBadStudents);
            Console.WriteLine("Average mark among good students is {0:f4}", 1.0 * sumGoodMarks / (nGoodStudents * nExams));
        }
    }
}



3.12
using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true) {
                Console.WriteLine("Enter r (or '-1' to exit)");
                double r = double.Parse(Console.ReadLine());
                if (r == -1) {
                    break;
                }
                Console.WriteLine("Do you want to calcullate area of 'square', 'circle', or 'triangle'?");
                string type = Console.ReadLine();
                double s = -1;
                if (type == "square") {
                    s = r * r;
                } else if (type == "circle") {
                    s = Math.PI * r * r;
                } else if (type == "triangle") {
                    s = 0.5 * Math.Sin(Math.PI / 3) * r * r;
                }
                Console.WriteLine(s);
            }
        }
    }
}



3.13
using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true) {
                Console.WriteLine("Enter A, B (or '-1' to exit)");
                var input = Array.ConvertAll(Console.ReadLine().Split(), double.Parse);
                if (input[0] == -1) {
                    break;
                }
                double a = input[0], b = input[1];

                Console.WriteLine("Do you want to calcullate area of 'rectangle', 'ring', or 'triangle'?");
                string type = Console.ReadLine();
                double s = -1;
                if (type == "rectangle") {
                    s = a * b;
                } else if (type == "ring") {
                    s = Math.PI * Math.Abs(a * a - b * b);
                } else if (type == "triangle") {
                    double angle = Math.Acos((0.5 * a) / b);
                    s = 0.5 * Math.Sin(angle) * a * b;
                }
                Console.WriteLine(s);
            }
        }
    }
}