private void exercicio5ToolStripMenuItem_Click(object sender, EventArgs e)
{
Form fc = Application.OpenForms["frmExercicio5"];
if (fc != null)
{
fc.Close();
}
frmExercicio5 frm = new frmExercicio5();
frm.MdiParent = this;
frm.WindowState = FormWindowState.Maximized;
frm.Show();
}