I have a long formula expressed as a string

 formula_1 = "6366.00 * Math.acos(Math.cos(self.latitude) * 
Math.cos(latitude) * Math.cos(longitude -self.longitude) + 
(Math.sin(self.latitude) * Math.sin(LatitudeRad)) )"

I need to use it in the preparation of some SQL query conditions like :

condition += (c(:villes){ 6366.00 * Math.acos(Math.cos(self.latitude) * 
Math.cos(latitude) * Math.cos(longitude -self.longitude) + 
(Math.sin(self.latitude) * Math.sin(LatitudeRad)) ) <  distance } )

How could I write it only once 'formula_1'  and reuse it  like :
condition += (c(:villes){ formula_1 <  distance })  # this is wrong as 
formula_1 is a string ..

thansk for your suggestions

joss