9.9×9.9+19.9 简便计算

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9.9×9.9+19.9 简便计算
9.9×9.9+19.9 简便计算

9.9×9.9+19.9 简便计算
9.9×9.9+19.9
=9.9×(10-0.1)+19.9
=9.9×10-9.9×0.1+19.9
=99-0.99+19.9
=99+18.91
=117.91

(10-0.1)(10-0.1)+(20-0.1),然后拆括号