Skill: Identify perpendicular lines

Perpendicular lines are lines that meet to make a right angle. A right angle measures $90^\circ$. The lines do not have to be the same length, and they can be horizontal, vertical, or slanted.

To identify perpendicular lines:

1. Find where the lines meet.
2. Look at the angle they form.
3. If the angle is a square corner, the lines are perpendicular.
4. If they do not meet, they are not perpendicular. If they meet but do not make a right angle, they are simply crossing lines.

Example

Look at rectangle $ABCD$:

A -------- B
|          |
|          |
D -------- C

Which sides are perpendicular to side $\overline{AB}$?

• $\overline{AB}$ meets $\overline{BC}$ at point $B$.
• The corner at $B$ is a right angle, so $\overline{AB} \perp \overline{BC}$.
• $\overline{AB}$ also meets $\overline{AD}$ at point $A$.
• The corner at $A$ is a right angle, so $\overline{AB} \perp \overline{AD}$.

Therefore, the sides perpendicular to $\overline{AB}$ are $\boxed{\overline{BC}\text{ and }\overline{AD}}$. The side $\overline{CD}$ is parallel to $\overline{AB}$ because they do not meet.