Hi

Relative density of a substance is defined is the ratio of density of the substance to the density of water at 4 degree Celsius.
Thus,
Relative density
= density of the substance/ density of water at 4 degree Celsius .

Formula of Linear Approximation is provided below:-

L(x) = f (a) + f' (a) (x - a) , this is called the Linearization of 'f' at 'a'.
Thus, Linear approximation formula is given following equation
f(x) f (a) + f' (a) (x - a)

Thankyou