### Theory:

Let us recall what a line, a line segment, a ray and an angle is.
Line:
If two points are extended endlessly in both directions in a straight path, a line is drawn.

In the figure, the two points $$P$$ and $$Q$$ are extended endlessly in both directions to get a line $$PQ$$.

The line $$PQ$$ is denoted as $$\overleftrightarrow{PQ}$$.
Line Segment:
The line connecting of two end points is a line segment.

In the figure, the line connects the two points end $$P$$ and $$Q$$ to form a line segment.

The line segment $$PQ$$ is denoted as $$\overline{PQ}$$.
Ray:
A line consisting of one end point or a starting point is a ray.

In the figure, the line $$PQ$$ extends endlessly from a starting point $$P$$ to form a ray.

The ray $$PQ$$ is denoted as $$\overrightarrow{PQ}$$.
Angle:
When two line segments or lines intersect with each other, they form an angle.

In the figure, the lines $$\overleftrightarrow{OA}$$ and $$\overleftrightarrow{OB}$$ intersect at $$O$$ to form an angle $$AOB$$.

The angle $$AOB$$ is represented as $$\angle AOB$$.

Important!