Question

in a formal letter do we use
"respected sir/man" or do we use
" dear sir/man" or simply
" sir/mam " for the salutation

Answer 1

I think, most often, we use respected sir or mam

Nd commonly we use word dear in informal letter such as dear brother etc.

Answer 2

you can use respected sir or mam or simply sir or mam.

but in letter to principal you can only use respected sir or mam.