$H=\frac{10}{56}+$$\frac{10}{140}+$$\frac{10}{260}+…$$\frac{10}{1400}$

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1. Đáp án:    `H = 5/14`

   Giải thích các bước giải:

   `H = 10/56 + 10/140 + 10/260 + … + 10/1400`

   `= 5/28 + 5/70 + 5/130 + … + 5/700`

   `= 5/(4.7) + 5/(7.10) + 5/(10.13) + … + 5/(25.28)`

   `= 5/3 . (3/(4.7) + 3/(7.10) + 3/(10.13) + … + 3/(25.28))`

   `= 5/3 . (1/4 – 1/7 + 1/7 – 1/10 + 1/10 – 1/13 + … + 1/25 – 1/28)`

   `= 5/3 . (1/4 – 1/28)`

   `= 5/3 . (7/28 – 1/28)`

   `= 5/3 . 3/14`

   `= 5/14`

    

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2. H=`10/56`+`10/140`+…+`10/1400`

   H=`5/28`+`5/70`+…+`5/700`

   H=`5/(4*7)`+`5/(7*10)`+…+`5/(25*28)`

   H=`1/3`*(`5/4`-`5/7`+`5/7`-`5/10`+…+`5/25`-`5/28`

   H=`1/3`*(`5/4`-`5/28`)

   H=`1/3`*`15/14`

   H=`5/14`

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