用换底公式,原式=[(1/2)(lg2*/lg5)*(4lg3)/(2lg7)]/{[lg(1/3)/(2lg5)]*[(2/3)lg2/lg7}=(2lg2*lg3)/2lg5lg7)*[(-2lg3lg5/3)/(2lg5lg7)=2lg2lg3lg5lg7/[(2lg5lg7/3)(-lg3lg2)]=-3.