99写为100-1
9写为10-1
再乘
=(100-1)*9
=100*9-1*9
=900-9
=891
99×9
=(100-1)×9
=900-9
=891
99*9
=9*(100-1)
=900-99
=891
1. 99×9
=(100-1)×9
=100×9-9
=900-9
=891
2. 99×9
=99×(10-1)
=99×10-99
=990-99
=891
3. 99×9
=(100-1)×(10-1)
=100×10-100-10+1
=1000-100-10+1
=891