4.5×3.9×0.2＝4.5×0.2×3.9＝0.9×3.9＝（4-0.1）×0.9＝3.6-0.9＝2.7
4.5×3.9×0.2=4.5×0.2×3.9=0.9×3.9=3.51
把小数点都提出来0.001×45×39×245×2=9090×39=3510再×0.001=3.51
