6.8.8 Condition for coplanarity of two lines

. . Condition for coplanarity of two lines (a) Condition in vector form The two given non-parallel lines r sb and r td are coplanar. So they lie in a single plane. Let A and C be the points whose position vectors are a  and c  . Then A and C lie on the plane. Since b and d are parallel to the plane, b is perpendicular to the plane. So AC is perpendicular to b . That is,