Question

mathematical form of relative density

Answer 1

relative density of a substance is refered to as the ratio of the desity of a subtsnce to the substance of water at 4°C
   Mathematical fORMULAE
      desity of a sustance/density of water

Answer 2

Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water.
Mathematical form will be :
Relative density = Density of given material/Density of reference material.