Geometry is full of
lines and curves or circles.
A straight line
is one of the basic terms in geometry.
Most of us say, "A line is
something straight that we draw with a ruler. It has a beginning
and an end." Yes, but in geometry, a line extends forever in
both directions.
Line Segments
Lines go forever. Line segments have a beginning and an
end. In geometry, line segments (pieces or parts) have names.
A segment is the part of the line between two points. We write the
name of a line passing through two different points, A and B, as
"line AB" or as
.
The arrows over AB mean that a line is passing through points A and
B.
Example:
The following is a diagram of two lines: line AB and line CD
The Dots
In geometry, the dots or
points just tell you where the letters are. They don't mean anything
geometric. They just show location.
Intersections
Sometimes, segments
meet as they cross each other. The point where they meet each other,
is called and intersection. That is like two streets that cross each
other at the intersection.
Line CD intersects
AB at G.
Example: Line 1
intersects the circle at point G. Line 2 intersects the rectangle at
points D and C.
Rays
Sometimes in geometry
you will see a line that starts at a certain point but goes forever.
That type of line is called a ray. The point where a ray
begins is called an end point.
A line segment has two
end points. A ray has one.
If you are referring to
ray AB, you would use the following symbol:
Parallel Lines
Two lines next to each other which never intersect are called
parallel lines. If line 1 is parallel to line 2, we write this as
line 1 || line 2
When two line segments, DC and AB, lie on parallel lines, we
write this as
segment DC || segment AB.
Example:
Lines 1 and 2 below are parallel.
Segments AB and CD below are parallel
A square and a rectangle have parallel sides. Their
opposite sides never meet.
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