A straight r is perpendicular to a plane if and only if r is perpendicular to all lines of passing through the intersection point of r and .
if a straight r is perpendicular to a plane then it is perpendicular or orthogonal to the entire straight line of :
- so that a straight r be perpendicular to a plane , it is sufficient to be perpendicular to two competing straights, contained in :
Notice in the picture below why it is not enough that r be perpendicular to a single line t in so that it is perpendicular to the plane:
Next: Relative Positions of Two Planes