title says it all... Im confused i don't get why...

The is wrong because when it says ALL, it means that EVERY symmetric line is a diagonal.

This can be proved wrong with a counterexample.

This here is a symmetric line, but not a diagonal

oh i meant that the title explains the question but thanks CU :)

A diagonal is just a line from a vertex to any other vertex.

Try it with a normal peice of rectangular paper.

Fold it along the diagonal.

Does it fold exactly in half and do the 2 sides overlap each other perfectly?

If not, then that diagonal is NOT an axis of symmetry.

i figured it out,,, but that's an amazing way to do it too thanks melody :))