The condition that the straight lines x=a1y+b1;  z=c1y+d1and x=a2y+b2;  z=c2y+d2 are perpendicular, is

The given lines rewrite in symmetric form as  x−b1a1=y=z−d1c1 and x−b2a2=y=z−d2c2The above two lines are perpendicular, hence  a1a2+1+c1c2=0