Geometry / Unit 3: Parallel and Perpendicular Lines
3.1 Lines and Planes in Space
Notes
Lines can relate to each other in three ways.
- Parallel lines are coplanar and never intersect: $\ell \parallel m$.
- Intersecting lines cross at one point. Perpendicular lines intersect at right angles: $\ell \perp m$.
- Skew lines are not coplanar; they never meet and are not parallel.
- Parallel planes never intersect.
- Example: in a box, a top front edge and a bottom side edge that point in different directions are skew.
Practice
Two lines do not intersect and are not in the same plane. They are:
Skew lines are noncoplanar and never meet.
What must be true about two parallel lines?
Parallel lines are coplanar and never intersect.
Line $\ell$ is perpendicular to line $m$. What is the measure of each angle they form?
Perpendicular lines form four right angles.
Ready for more? Take the Unit 3 test.