原式＝（8-0.8）×4.5+5.5＝8×4.5-0.8×4.5+5.5＝36-3.6+5.5＝32.4+5.5＝37.9
7.2*4.5+5.5=7.2（5-0.5）+5.5=36-3.6+5.5=36+（5.5-3.6）=36+（5.5-3.5-0.1）=36+（2-0.1）=36+1.9=37.9这样计算就简便点
