实现效果:

知识运用:
窗体的IsMdiContainer MdiParent 和 WindowState属性
实现代码:
private void button1_Click(object sender, EventArgs e)
{
this.IsMdiContainer = true;
Form2 frm2 = new Form2();
frm2.MdiParent = this;
frm2.WindowState = FormWindowState.Maximized;
frm2.Show();
}