Show that the straight lines whose direction cosines are given by the equations `al+bm+cn=0` and `u l^2+z m^2=v n^2+w n^2=0` are parallel or perpendicular as `a^2/u+b^2/v+c^2/w=0` or `a^2(v +w)+b^2(w+u)+c^2(u+v)=0`