replacing a formula from a string

[Josselin]

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 += (cvilles){ 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 += (cvilles){ formula_1 < distance }) # this is wrong as
formula_1 is a string ..

thansk for your suggestions

joss