Thanks. After a bit of experimentation, I found that you can have a block outputting several lines, but you need to manually add commas and carriage returns, e.g.,

#+BEGIN_SRC mathematica :results output
f[x_] := 3*x^2+3;
f[8], "\n",
D[f[x],x]
#+END_SRC

#+RESULTS:
: 195
: 6*x





> On Mar 13, 2015, at 4:16 PM, Yi Wang <tririverwangyi@gmail.com> wrote:
> 
> Dear Richard,
> 
> You can do:
> 
> #+BEGIN_SRC mathematica
> f[x_] := x+3;
> f[5]
> #+END_SRC
> 
> Here are explanations: I actually put everything in the SRC block into a Print[...].
> 
> So in the second example, what actually runs is
> 
> Print[f[x_] := x+3
> f[5]]
> 
> This does not return any value. Because what it really does is print the value of "f[x_] := x+3 f[5]"
> 
> Here is the code:
> 
> (concat
>      (mapconcat ;; define any variables
>       (lambda (pair)
> 	(format "%s=%s;"
> 		(car pair)
> 		(org-babel-mathematica-var-to-mathematica (cdr pair))))
>       vars "\n") "\nPrint[\n" body "\n]\n")
> 
> If anybody has idea to improve it, I will be happy to see and work it out!
> 
> Best,
> Yi
> 
> 
> 
> 
> On Sat, Mar 14, 2015 at 7:09 AM John Kitchin <jkitchin@andrew.cmu.edu <mailto:jkitchin@andrew.cmu.edu>> wrote:
> I wonder if this is an output vs value issue on the return of the
> block. If you set :results output does anything change? or, is there a
> way to specify a return value? or specifically print something?
> 
> Richard Stanton writes:
> 
> > I recently saw this posting about org support for Mathematica. Thanks for writing this!
> >
> > Unfortunately, while I can get it to work for simple things (e.g., 2+3), even slightly more complex things don't seem to work. For example, suppose I'd like to define a function f(x) = x + 3, and then calculate f(5). In a MMA notebook, if I type
> >
> > f[x_] := x+3
> > f[5]
> >
> > I get the answer 8. If I try the same code in an org-mode code block, here's what happens:
> >
> > #+BEGIN_SRC mathematica
> > f[x_] := x+3
> > f[5]
> > #+END_SRC
> >
> > #+RESULTS:
> > : Null
> >
> > Am I missing something obvious here?
> >
> > Thanks very much.
> >
> > Richard Stanton
> 
> --
> Professor John Kitchin
> Doherty Hall A207F
> Department of Chemical Engineering
> Carnegie Mellon University
> Pittsburgh, PA 15213
> 412-268-7803
> @johnkitchin
> http://kitchingroup.cheme.cmu.edu <http://kitchingroup.cheme.cmu.edu/>