private void button3_Click(object sender, EventArgs e)
 {
     Phan5.HinhHoc.Bai3 fr = new Phan5.HinhHoc.Bai3();
     fr.ShowDialog(this);
 }
 private void button4_Click(object sender, EventArgs e)
 {
     Phan5.HinhHoc.Bai3 fr = new Phan5.HinhHoc.Bai3();
     fr.Show();
 }