$6-2$ Show that, for a constant$-$composition PVT system,

$d{}_i=-(S{)}_{i}dT+(V{)}_{i}dP$

Where $(S{)}_i=[\frac{\text{d}\text{e}\text{l}(ns)}{del{n}_i}{]}_{T,P,n_j}i$

$(V{)}_i=[\frac{\text{d}\text{e}\text{l}(nV)}{del{n}_i}{]}_{T,P,n_j}i$