线段全等和等长

Two line segments are congruent if they have the same length. They do not need to have the same position or orientation.

In the diagram above, segments $\overline{AB}$ and $\overline{CD}$ are congruent to each other and segments $\overline{EF}$ and $\overline{EG}$ are congruent to each other. We say that $\overline{AB}\cong\overline{CD}$ (this is read as "line segment AB is congruent to line segment CD") and $\overline{EF}\cong\overline{EG}$. However, congruency of line segments differs from equality of line segments in the sense that equality compares distance. For example, $AB=CD$ because $AB$ and $CD$ represent the length of the lines.

(Video) Can Line Segments be Congruent?

by eHow

Here is a video explaining line segment congruence

Line segments are congruent if they have the same length, or measure. When we previously talked about congruence, we said that for two geometric figures to be congruent they have to be the same shape and size. All line segments have the same shape, so "size" is what we are really looking at when we want to know if two line segments are congruent.