private void Sub_Click(object sender, EventArgs e)
{
   rational ra1 = new rational();
   rational ra2 = new rational();
   rational ra3 = new rational();
   ra1.x = System.Convert.ToInt16(textBox1.Text, 10);
   ra1.y = System.Convert.ToInt16(textBox2.Text, 10);
   ra2.x = System.Convert.ToInt16(textBox3.Text, 10);
   ra2.y = System.Convert.ToInt16(textBox4.Text, 10);
   ra3.sub(ra1, ra2, ra3  );
   label5.Text = "Subtract is " + ra3.x.ToString() + "/" + ra3.y.ToString();
}
