Как вывести построчно в RichTextBox?

Как мне вывести результат вычислений в несколько строк, чтобы вывелось допустим:

Δ1=1

Δ2=1

Δ3=1

Δ4=1

А не просто последнее значение Δ4=1. Вот листинг программы:

            double x1 = Convert.ToDouble(textBox1.Text);
            double x2 = Convert.ToDouble(textBox2.Text);
            double x3 = Convert.ToDouble(textBox3.Text);
            double x4 = Convert.ToDouble(textBox4.Text);
            double dx1 = Convert.ToDouble(textBox5.Text);
            double dx2 = Convert.ToDouble(textBox6.Text);
            double dx3 = Convert.ToDouble(textBox7.Text);
            double dx4 = Convert.ToDouble(textBox8.Text);
            double s1 = Convert.ToDouble(textBox9.Text);
            double s2 = Convert.ToDouble(textBox10.Text);
            double s3 = Convert.ToDouble(textBox11.Text);
            double s4 = Convert.ToDouble(textBox12.Text);
            double ls = Convert.ToDouble(textBox13.Text);
            double l = x1 + x2 + x3 - x4;
            if (l > ls)
            {
                double k = 1/(1 / (s1 * s1) + 1 / (s2 * s2) + 1 / (s3 * s3) + 1 / (s4 * s4));
                double p1 = k / (s1 * s1);
                double p2 = k / (s2 * s2);
                double p3 = k / (s3 * s3);
                double p4 = k / (s4 * s4);
                double numerator = 2*p1 + 2*p2 + 2*p3 + 2*p4;
                double denominator = numerator / (2 * p1) + numerator / (2 * p2) + numerator / (2 * p3) + numerator / (2 * p4); 
                double lambda = -1 * numerator * l / denominator;
                double ddx1 = -1 * lambda / (2 * p1);
                double ddx2 = -1 * lambda / (2 * p2);
                double ddx3 = -1 * lambda / (2 * p3);
                double ddx4 = lambda / (2 * p4);
                    if (Math.Abs(ddx1) <= dx1)
                    {
                        richTextBox1.Text = "Δx1: " + ddx1 + " <= " + dx1;
                    }
                    else richTextBox1.Text = "Δx1: " + ddx1 + " > " + dx1 + " - отказ";
                    if (Math.Abs(ddx2) <= dx2)
                    {
                        richTextBox1.Text = "Δx2: " + ddx2 + " <= " + dx2;
                    }
                    else richTextBox1.Text = "Δx2: " + ddx2 + " > " + dx2 + " - отказ";
                    if (Math.Abs(ddx3) <= dx3)
                    {
                        richTextBox1.Text = "Δx3: " + ddx3 + " <= " + dx3;
                    }
                    else richTextBox1.Text = "Δx3: " + ddx3 + " > " + dx3 + " - отказ";
                    if (Math.Abs(ddx4) <= dx4)
                    {
                        richTextBox1.Text = "Δx4: " + ddx4 + " <= " + dx4;
                    }
                    else richTextBox1.Text = "Δx4: " + ddx4 + " > " + dx4 + " - отказ";;```