real function fx(x,y)

c     Return the time-derivative of x in terms of x and y.  Modify this
c     to the one for your problem.

c     Warning: do not divide by integers; add a decimal point to make
c     them floating point numbers.

c     avoid typos
      implicit none

c declarations

c     function arguments
      real x,y

c executable

c     by definition x = theta and y = d(theta)/d(t), so d(x)/d(t) = y:
      fx=y

      return
      end