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| 3×(8×7)×125, =(3×7)×(8×125), =21×1000, =21000; 101×99, =(100+1)×99, =100×99+99, =9900+99, =9999; 37+37×39, =37×(39+1), =37×40, =1480; 32×25, =8×(4×25), =8×100, =800; 201×42-42, =(201-1)×42, =200×42, =8400; 199+199×99, =199×(99+1), =199×100, =19900. |
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a.不变
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a.扩大到原来的2倍
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a.相交