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  爆急1/3+1/9+1/27+…+1/243+1/7291+1/3-7/12+9/20-11/30+13/42-15/56+17/721993x19951995-1995x19931993注意巧算,答对给你10分以上

  爆急

  1/3+1/9+1/27+…+1/243+1/729

  1+1/3-7/12+9/20-11/30+13/42-15/56+17/72

  1993x19951995-1995x19931993

  注意巧算,答对给你10分以上

1回答
2020-04-2810:50
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梁晓龙

  1/3+1/9+1/27+…+1/243+1/729

  =(1/3+1/9+1/27+…+1/243+1/729)×2÷2

  =(2/3+2/9+2/27+…+2/243+2/729)÷2

  =(2/3+2/9+2/27+…+2/243+2/729+1/729-1/729)÷2

  =(2/3+2/9+2/27+…+2/243+1/243-1/729)÷2

  =(1-1/729)÷2

  =728/729×1/2

  =364/729

  1+1/3-7/12+9/20-11/30+13/42-15/56+17/72

  =1+1/3-(3+4)/(3×4)+(4+5)/(4×5)-(5+6)/(5×6)+(6+7)/(6×7)-(7+8)/(7×8)+(8+9)/(8×9)

  =1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+1/8+1/9

  =1+1/9

  =10/9

  1993x19951995-1995x19931993

  =1993×1995×10001-1995×1993×10001

  =0

2020-04-28 10:51:22
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