The angle between the diagram of a parallelogram whose sides are lx+my+n=0; lx+my+n1=0; mx+ly+n=0; mx+ly+n1=0 is
Show that the diagonals of the parallelogram whose sides are lx+my+n=0, lx+my+n′=0, mx+ly+n=0 and mx+lu+n′=0 include an angle π2