A line perpendicular to a plane

• A line is perpendicular to a plane if it is perpendicular to two intersecting straight lines lying in that plane.
• This condition is used often to prove perpendicularity: if a line is perpendicular to two intersecting lines in a plane, then it is perpendicular to the whole plane.

A line parallel to a plane

1. A line is parallel to a plane if it is parallel to a straight line lying in that plane
2. and it does not lie in the plane.

1. Two parallel planes never meet, however far they extend.
2. Two intersecting planes meet in one straight line.

Distance from a point to a plane

• The distance from a point to a plane is the length of the perpendicular drawn from the point to the plane, that is the shortest distance between them.