1.999^2-998^2 =(999-998)*(999+998)=1*(1000-1+1000-2)=2000-3=19972.(81又1/2)^2-(78又1/2)^2=(81+1/2)^2-(78+1/2)^2=(81+1/2-78-1/2)(81+1/2+78+1/2)=3*(159+1)=3*160=480