Задача 2 (Стр. 31, Зад. 14):

namespace arithmetic_progression
{
    using System;

    class ArithmeticProgression
    {
        static void Main(string[] args)
        {
            double aFirst = double.Parse(Console.ReadLine());
            double differense = double.Parse(Console.ReadLine());
            int n = int.Parse(Console.ReadLine());

            double aN = aFirst + (n - 1) * differense;

            double sum = (aFirst + aN) * n / 2;

            Console.WriteLine(sum);
        }
    }
}