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`C = 1.5 + 2.6 + 3.7 +...+26.30`

   `= 1.(3+2) + 2.(3+3) + 3.(3+4)+...+26.(3+27)`

   `= 1.3 + 1.2 + 2.3 + 2.3 + 3.3 + 3.4 +...+ 26.3 + 26.27` 

   `= ( 1.3 + 2.3 + 3.3 +...+26.3 ) + ( 1.2 + 2.3 + 3.4 +...+26.27 ) `

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Đặt `X = 1.3 + 2.3 + 3.3 +...+26.3`

          `= 3.( 1 + 2 + 3 +...+26 )`

          `= 3.(26 + 1) . 26 : 2`

          `= 3. 27 . 26 : 2`

          `= 1053`

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Đặt `A = 1.2 + 2.3 + 3.4 +...+26.27`

     `3A = 1.2.3 + 2.3.3 + 3.4.3 +...+ 26.27.3`

     `     = 1.2.3 + 2.3.( 4 - 1) + 3.4.(5-2) +...+ 26.27.(28-25)`

     `     = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +...+ 26.27.28 - 25.26.27`

     `     = 26.27.28 `

  `⇒ A = ( 26 . 27 . 28)/3 = 26 . 9 .28 = 6552`

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Ta có : `C = X + A`

`   = 1053 + 6552`

`   = 7605`

Vậy `C = 7605`