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98x199用巧算怎么做

原式=98×(200-1) =98×200-98×1 =19600-98 =19600-100+2 =19500+2 =19502

0.98X2.5 =0.49X2X2.5 =0.49X5 =0.5X5-0.01X5 =2.5-0.05 =2.45

(980×65-320)÷(66+98×64) =(980×64+980-320)÷(98×64+66) =(980×64+660)÷(98×64+66) =(98×10×64+10×66)÷(98×64+66) =10×(98×64+66)÷(98×64+66) =10×1 =10

解: 用裂项的思想 1/12x13+1/13x14+1/14x15+…+1/98x99+1/99x100 =(1/12-1/13)+(1/13-1/14)+...+(1/98-1/99)+(1/99-1/100) (中间的两两消去了) =1/12-1/100 =11/150

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