Theory:

The line that dissects the image into two identical halves is called the line of symmetry or the axis of symmetry.
2(1) Ресурс 1.png
 
In the figure given above, the line that runs through is called the line of symmetry. This line dissects the image into two identical halves.
 
The line of symmetry can be vertical, horizontal or diagonal.
 
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The same figure can also contain more than two or more lines of symmetry.
 
It means that the given figure can be dissected into two identical halves in two or more ways.
 
The lines that do not dissect the images into two identical halves are not considered as lines of symmetry.