I think no, now all lines of symmetry are diagonals because it can also be horizontal and vertical.
Symmetry is also known as proportion; anything that is equal or proportion with each other.
Let’s have an example:
A square
=> in getting the symmetry of a square, we need to draw a 2 diagonal lines to each edges.
=> we can also draw 2 perpendicular lines that bisect to the edges of the square and it also provide an equal parts or proportional parts.
i just looked it up kk ;
Not all lines of symmetry are diagonals. Lines of symmetry can be vertical, horizontal, or diagonal, whereas diagonals specifically connect non-adjacent vertices. For example, a square has several lines of symmetry, only two of which are diagonal.
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