линейная интерполяция (версия 1)

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

namespace razrabotka
{
    class Program
    {
        static void Main(string[] args)
        {

            string[] buf = new string[256];
            var x = new List<float>();
            var f = new List<float>();

            System.IO.StreamReader file =
                new System.IO.StreamReader(args[0]);
            while ((  buf[0] = file.ReadLine()) != null)
            {
                buf = buf[0].Split(' ');
                foreach (var s in buf)
                {

                    string temp = s;
                    if(temp.Contains('f'))
                    {
                        temp = temp.Remove(temp.IndexOf('f'), 6);
                        temp = temp.Remove(temp.IndexOf(';'), 1);
                        float tmpf = 0;
                        float.TryParse(temp, out tmpf);
                        f.Add(tmpf);
                    }
                    else  if (temp.Contains('x'))
                    {
                        temp =  temp.Remove(temp.IndexOf('x'), 3);
                        temp = temp.Remove(temp.IndexOf(';'), 1);
                        float tmpx = 0;
                        float.TryParse(temp, out tmpx );
                        x.Add(tmpx);
                    }
                }
            }

            file.Close();

            float f2 = f[0] + ((f[1] - f[0]) * (x[1] - x[0])) / (x[2] - x[0]);
            double k = (f[1] - f[0]) / (x[2] - x[0]);
            double b = (x[2] * f[0] - f[1] * x[0]) / (x[2] - x[0]);

            Console.WriteLine("функия : f(x) = ({0})*x+({1});",k,b);
            Console.WriteLine("f({0})= {1}", x[1], f2);
        }
    }
}