Axis of symmetry for Axisymmetric 2D Solid 1

[SaeidII]

How do you define axis of symmetry for an Axisymmetric 2D solid? It seems there are some rules, but I did not find much information in documentations.
Thanks for helping me.

 

[ZeroExperience]

http://www.youtube.com/watch?v=dPN3CCc39Ik&list=PLI39KM_btemwwh_QEJWRpkrsK7EN7Kmth&index=6

http://www.youtube.com/watch?v=Qi4YktKaJfs&list=PLI39KM_btemwwh_QEJWRpkrsK7EN7Kmth&index=8

 

[SaeidII]

Thank you Z.E, very helpful.