How to derive variation of "g" …

let us consider an object is placed at a point having mass m below the surface of the earth at the surface of earth g=GM/R^2 let at point a acceleration due to gravity be g g=GM/R^2. ..........eq.1 let mass M=p(density)×V(volume) M=p×4/3πR^3 put the value of M in eq. 1 g=Gp×4/3πR^3/R^2 g=Gp×4/3πR. ........eq.2 let at point b the value of g is g' g'=GM/(R-x)^2. .......eq.3 let mass M= p×V M=p×4/3π(R-x)^3. put the value of M in eq. 3 g'=Gp×4/3(R-x)^3/(R-x)^2 g'=Gp×4/3(R-x) .........eq.4 divide eq.4 by eq.2 g'/g=Gp×4/3π(R-x)/Gp×4/3πR g'/g=R-x/R g'/g=(1-x/R) g'=g(1-x/R)