bài 1: a]4/20+16/42+6/15+-3/5+2/21+-10/21+3/20 b]42/46+250/185+-2121/2323+-125125 bài 2: 1/1.3+1/1.5+1/5.7+……+2007.2009 giúp mình với cho 5 sao 1

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1. Bai 1

   a4/20+16/42+6/15+-3/5+2/21+-10/21+3/20

   =1/5+8/21+6/15_+ -3/5 +2/21+-10/21+3/20

   =(1/5+6/15-3/5)+(8/21+2/21+-10/21)+3/20

   =(1/5+6/15-3/5)+0+0+3/20

    =0+0+0+3/20

   =3/20

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