【简便算法①】:
81.9×0.9-0.9×1.9
=(81.9-1.9)×0.9
=80×0.9=8×9
=72
【简便算法②】:
81.9×0.9-0.9×1.9
=(80+1.9)×0.9-1.9×0.9
=80×0.9+1.9×0.9-1.9×0.9
=72
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