Section 2.2

Also includes perpendicular and parallel lines

Perpendicular and Parallel Perpendicular lines intersect to form a right angle

A line that is perpendicular to a plane is perpendicular to every line in that plane that it intersects

Definitions, such as these, are meant to be interpreted in both directions.

For example, the first definition could be written

“Two lines that form right angles are perpendicular”

Biconditional Statements Like definitions, biconditional statements can be

interpreted in both directions

They are written in the form:

<1st statement> if and only if <2nd statement>

“If and only if” is often abbreviated “iff”

One can make a biconditional statement if the conditional statement and its converse are both true (or both false, but why would you do that)

Reference McDougal Littel: Geometry, Section 2.2