For Liliya1

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

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Введите комплексные числа: ");
            Console.WriteLine("Введите комплексное число 1: ");
            double a1 = Convert.ToDouble(Console.ReadLine());
            double b1 = Convert.ToDouble(Console.ReadLine());
            Console.WriteLine("Введите комплексное число 2: ");
            double a2 = Convert.ToDouble(Console.ReadLine());
            double b2 = Convert.ToDouble(Console.ReadLine());

            Console.WriteLine("Выберите действие: ");

            int change = Convert.ToInt32(Console.ReadLine());

            Console.WriteLine(ComplexNumb(a1, b1, a2, b2, change)[0] + "; " + ComplexNumb(a1, b1, a2, b2, change)[1] + "i");

            Console.ReadKey();
        }

        public static double[] ComplexNumb(double a1, double b1, double a2, double b2, int vibor)
        {
            double[] temp = new double[2];
            switch (vibor)
            {
                case 1:
                    temp[0] = a1 + a2;
                    temp[1] = b1 + b2;
                    break;
                case 2:
                    temp[0] = a1 - a2;
                    temp[1] = b1 - b2;
                    break;
                case 3:
                    temp[0] = a1 * a2 - b1 * b2;
                    temp[1] = a1 * b2 + a2 * b1;
                    break;
                case 4:
                    temp[0] = (a1 * a2 + b1 * b2) / (a2 * a2 + b2 * b2);
                    temp[1] = (a2 * b1 - a1* b2) / (a2 * a2 + b2 * b2);
                    break;
            }
            return temp;
        }
    }
}