private void bài19ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Bai19 bai19 = new Bai19();

            bai19.MdiParent = this;
            bai19.Show();
        }
        public MDIParent1()
        {
            InitializeComponent();
            Bai19 bai19 = new Bai19();

            bai19.MdiParent = this;
            bai19.Show();
            Bai19 bai191 = new Bai19();

            bai191.MdiParent = this;
            bai191.Show();
            Bai19 bai192 = new Bai19();

            bai192.MdiParent = this;
            bai192.Show();
        }
        private void button13_Click(object sender, EventArgs e)
        {
            Bai19 bai19 = new Bai19();

            bai19.Show();
        }