tính nhanh : 7/6+7/12+7/20+7/30+7/42+7/56= ?

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1. `7/6+7/12+7/20+7/30+7/42+7/56`

   `=7/2.3+7/3.4+7/4.5+7/5.6+7/6.7+7/7.8`

   `=7(1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8)`

   `=7(1/2-1/3+1/3-1/4+1/41-1/5+1/5-1/6+1/6-1/7+1/7-1/8)`

   `=7(1/2-1/8)`

   `=7(4/8-1/8)`

   `=7 . 5/8`

   `=35/8`

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2. Đáp án:21/8

    

   Giải thích các bước giải:

   7/6 + 7/12 + 7/20 + 7/30 + 7/42 +7/56

   =7(1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56

   =7(1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)+1/(7*8))

   =7(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8)

   =7(1/2-1/8)

   =7*(3/8)=21/8

    

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