0.54×9.9=0.54×(10-0.1)=5.4-0.054=5.346
0.54×9.9=0.54×(10-0.1)=54-0.54=53.46
0.54x9.9=0.54x（1-0.1）=0.54-0.054=0.486
(0.55-0.01)×(1-0.1)
