=9.9x4.5+0.1x4.5=（9.9+0.1）x4.5=45
9.9×4.5+4.5×0.1=(9.9+0.1)×4.5=10×4.5=45
=99×0.45+0.45=0.45×（99+1）=0.45×100=45
