Light charged particles emitted from the reactions of 35MeV/u 40Ar+197Au are measured experimentally. A new observable,R=d/(d+t+3He+4He), which is defined as the reduced yield of deuterons, has a monotone relation with the entropy and it is independent of the breakup density (ρ/ρ0) and N/Z of the system. The entropy values are extracted by using several methods which use the d/p ratios,dlike/plike ratios, reduced yield of isotope deuterons R and the reduced multiplicity of charged particles Mred, respectively. The entropy values from the reduced yield of isotope deuterons R and the reduced multiplicity of charged particles Mred are nearly the same. These methods can be used in low beam energy heavy ion reactions. The charged particles emitted in rear angles are mainly from an equilibrium-state source with temperature about 4.7±1.2MeV, which are extracted from the double isotope yields ratios, and entropy S/A=2.5±0.5.This implies that the Break-up density is slightly less than 0.1(ρ/ρ0).