`Tính bằng cách hợp lí a/1+3+5+7+9+…+2013 b/100+98+96+…+2-97-95-…-1 c/1+2+3+4+…+n d/2.3.12+4.6.42+8.27.3

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1. a) 1+3+5+7+…+2013

   =(2013+1)+(2011+3)+…+(1009+1005)+1007

   =2014+2014+…+2014+1007

   =2014*1007+1007

   =2015*1007

   =2 029 105

   b)100+98+96+…+2-97-95-…-1

   =(98-97)+(96-95)+…+(2-1)+100

   =1+1+1+…+1+100

   =1*49+100

   =49+100

   =149

   c)1+2+3+4+…+n

   =(n+1)+(n-1+2)+(n-2+3)+…

   =n+1+n+1+n+1+…

   d)2.3.12+4.6.42+8.27.3

   =(2*12)*3+(4*6)*42+(8*3)*27

   =24*3+24*42+24*27

   =24*(3+27+42)

   =24*72

   =1728

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