21×9×19×9
=(21×19)×9²
=[(20+1)×(20-1)]×81
=(20²-1²)×81
=(400-1)×81
=(400-1)×81
=400×81-1×81
=32400-81
=32319
很高兴为您解答,【the 1900】团队为您答题.
请点击下面的【选为满意回答】按钮.21×9×19×9=(21×9)×(19×9)=[(20+1)×9]×[(20-1)×9]=(20×9+1×9)×(20×9-1×9)=(180+9)×(180-9)=180²-9²=32400-81=32319