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巧算:1.3.9x2.8%8.9x2.9+3.9x6.1

1、 3.9×2.8-8.9×2.9+3.9×6.1 = 3.9×2.8+3.9×6.1-8.9×2.9 = 3.9×(2.8+6.1)-8.9×2.9 = 3.9×8.9-2.9×8.9 = (3.9-2.9)×8.9 = 1×8.9 = 8.9 2、 333×666+999×778 = 333×(3×222)+999×778 = 333×3×222+999×778 = 999×222+999×778 = 999×(222+778) = 999...

=2.9*(2.9+6.1+1) =2.9*10 =29

1×2+2×3+...+8×9 太长了,就不抄全了。 =1×(1+1)+2×(2+1)+...+8×(8+1) =1²+2²+...+8²+1+2+...+8 =8×9×17/6 +8×9/2 =204+36 =240 利用公式: 1+2+...+n=n(n+1)/2 1²+2²+...+n²=n(n+1)(2n+1)/6

9.3×(1+6.3+2.7)

原式等于 6×(14-8-1)=30 所以原式的计算结果为30.

7.6x3.2+7.6x6.7+0.76 =7.6x(3.2+6.7+0.1) =7.6x10 =76

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