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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...

没有简便算法: 7.2×3.9+2.8×6.1 =28.08+17.08 =45.16

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

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