4.5×9.9×0.45=4.5×（10-0.1）×0.45=4.5×10×0.45-4.5×0.1×0.45=4.5²-0.45²=（4.5+0.45）（4.5-0.45）=4.95×4.05=（5-0.05）（4+0.05）=20-0.2+0.25-0.0025=20.0475