double a = System::Convert::ToDouble(this->textBox3->Text);
        double b = System::Convert::ToDouble(this->textBox4->Text);
        double eps = (System::Convert::ToDouble(this->trackBar1->Value))*0.0001;
        double x = System::Convert::ToDouble(this->textBox5->Text);
        int n = 2;
        double step;
        double x1 = a, x2, x3, Fy,F1y;
        do
        {
            step = (b-a)/n;
            x2 = a + step;
            x3 = x2 + step;
            Fy = (y(x2) - y(x1))/(x2 - x1);
            F1y = (y(x3) - y(x2))/(x3 - x2);
        }
        while(fabs((F1y - Fy)) > eps);
        double dif = (y(x + step) - y(x))/((x+step) - x);
            this->label13->Text = System::Convert::ToString(dif);