grouping the variables in a certain way

Say I have

(%i5) a : D^2 * (D^2 - x^2)/(D+3*x)^4;
                                  2   2    2
                                 D  (D  - x )
(%o5)                            ------------
                                           4
                                  (D + 3 x)

How can I change 'a' so that maxima displays


(%i6) b : (1-(x/D)^2)/(1+3*(x/D))^4;
                                         2
                                        x
                                    1 - --
                                         2
                                        D
(%o6)                             ----------
                                   3 x     4
                                  (--- + 1)
                                    D

That is I want to simplify the expression and show the result in terms of
(x/D) instead of x,D.

Any ideas?

thanks
raju


-- 
Kamaraju S Kusumanchi
http://www.people.cornell.edu/pages/kk288/
http://malayamaarutham.blogspot.com/