Two lines are perpendicular if they intersect to form a right angle. A line perpendicular to a plane is perpendicular to every line it intersects in that plane. Definitions of geometric terms can be interpreted both ways, such as defining perpendicular lines as those that form right angles. Biconditional statements relate two statements that are logically equivalent, using "if and only if" or "iff", and are true when the statement and its converse are both true.
2. 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
3. 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)