把38看成1乘38，然后99*38+1*38=100*38=3800
=99*38+38=38(99+1)=38*100=3800
38x（99+1）=3800
99x38+38=(99+1)x38=100x38=3800
99*38+38=（99+1）*38=100*38=3800
