[plt-scheme] Understanding lambda

> Thanks for the help guys , so now i know that  in (define f (lambda(x) (+(*
> 3 x) 5)))  value of f is a function returned by the lambda expressions.so
> when i do ( f 1)  or anything as in (f 55) then what are 1 or 55  in this
> expression , constants or functions?

You can play around with some stuff in the REPL:

> (procedure? 1)
#f
> (procedure? f)
#t
> (number? 1)
#t
> (number? f)
#f