c$)$ Show that the energy needed to remove two neutrons from an odd$-$mass nucleus is approximately

$S_{2n}(A,Z)2[a_v-\frac{2}{3}{a}_s{A}^{-\frac{1}{3}}+\frac{1}{3}{a}_c{Z}^2{A}^{-\frac{4}{3}}-a_a(1-\frac{4Z^2}{A^2})],$

in the limit $A2$ so that $A$ in $M(A,Z)$ can be treated as a continuous variable.