private void bài21ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Bai21 bai21 = new Bai21();

            bai21.MdiParent = this;
            bai21.Show();
        }
        private void button11_Click(object sender, EventArgs e)
        {
            Bai21 bai21 = new Bai21();

            bai21.Show();
        }