Factorise : mx−ny+nx−my
If the lines lx+my+n=0, mx+ny+l=0 and nx+ly+m=0 are concurrent then (l,m,n≠0)
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