private void lịchSửHóaĐơnToolStripMenuItem_Click(object sender, EventArgs e)
        {
            fBillHistory f = new fBillHistory();

            this.Hide();
            f.ShowDialog();
            this.Show();
        }
        private void lBillHistory_Click(object sender, EventArgs e)
        {
            fBillHistory f = new fBillHistory();

            this.Hide();
            f.ShowDialog();
            this.Show();
        }