The vector is normal to the plane
and the vector is normal to a second plane .
Do the two planes necessarily intersect if they are both extended indefinetly?
If the two planes do intersect, find a vector which is parallel to their line of intersection.
Two planes are parallel if their normal vectors are parallel (equal).
. . Since , the planes will intersect.
Their line of intersection has the vector: .
. . Do you see why?
. . Therefore: .