3.9×3.9=(4-1)×(4-1)=4×4-1=152.85×5.35+2.85×4.65=2.85×(5.35+4.65)=2.85×10=28.56.05÷1.25÷0.8=6.05÷(1.25×0.8)=6.05÷1=6.05
简便方法如图所示