Question

Why does the term angle need to be defined prior to defining perpendicular lines?

Answer 1

Perpendicular lines are two or more lines that intersect at a 90-degree angle, like the two lines drawn on this graph, and the x and y axes that orient them.

Answer 2

Perpendicular means "at right angles".

A line meeting another line at a right angle, or 90° is said to be perpendicular to it.

This is why we need to know what are angles first.

Angles are defined as the space between two intersecting lines close to the point where they meet.

So, when we say perpendicular lines meet at 90° angles, we must know the definition of angles.