Can I extend actor in use case? for example I have "normal user" , "student user" and "teacher user"... every use case the "normal user" uses it ,by default the "student user" and "teacher user" use it too.. so can I extend "normal user" for "student user" and "teacher user" ? is that popular ? Thanks a lot :)
Yes, for example, you may have a Database Administrator and an Active Directory administrator.
You can factor out the common needs to a new base actor "System Administrator" for a simplified model.
You can represent the actor generalisation like any other, draw a solid line with a closed arrow pointing from the specialised actor to the base actor.