// точка вместо запятой в вещественных
            System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("en-US");

            richTextBox1.Clear();
            chart1.Series[0].Points.Clear();
            chart1.Series[1].Points.Clear();
            double x1, x2, step, foo1, foo2, tmp;
            x1   = Convert.ToDouble(textBox1.Text);
            x2   = Convert.ToDouble(textBox2.Text);
            step = Convert.ToDouble(textBox3.Text);
            for(double x = x1; x <= x2; x+=step)
            {
                tmp = x/3;
                foo1 = tmp*tmp;
                foo2 = Math.Cos(x + Math.PI / 3) + 1.8;
                chart1.Series[0].Points.AddXY(x, foo1);
                chart1.Series[1].Points.AddXY(x, foo2);
                richTextBox1.Text += "x = "    + x.ToString("0.000")   + "\t"
                                  +  "y(x) = " + foo1.ToString("0.00") + "\t"
                                  +  "z(x) = " + foo2.ToString("0.00") + "\n";


            }