1. Tính nhanh : a, 1 + 7 + 13 + … + 427 b, 17 + 25 + 33 + … x 737 c, 97 . 37 + 54 . 67 – 13 . 37 + 67 . 46 + 16 . 37

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1. `a. 1 + 7 + 13 + …. + 427` 

   ` = [(427-1):6+1]. (427+1)/2`

   `=15408`

   `b, 17+25+33+……+737`

   `=[(737-17):8+1].(737+1)/2`

   `= 33579`

   `c,97.37+54.67-13.37+67.46+16.37`

   `=37.(97-13+16)+67.(54+46)`

   `=37.100+67.100`

   `=100(37+67)`

   `=100 . 104 = 10400`

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