Perpendicular Lines

Two segments are perpendicular if and only if the lines containing them form a 90 degree angle. stands for "is perpendicular to". Again, there isn't that much to perpendicular lines. There are a few theorums that you should know, however.

Perpendicular Lines and Slopes Theorum: In a coordinate plane, the slopes of perpendicular lines are inverse reciprocals.
Perpendicular to Parallels Theorum: In a plane, if a line is perpendicular to one of two parallel lines, it's perpendicular to the other. See proof.
Two Perpendiculars Theorum: If two coplanar lines a and b are each perpendicular to the same line, then they are parallel to each other. See proof.

Note: While you can usually get away with not knowing the names of theorums, your Geometry teacher will generally require you to know them.

- Jaime III
Education is that which allows you to get into more intelligent trouble.