Cau 1 Tính nhanh nếu có thể a) -8/18 – 15/27 b) -3/2 : 9/4 c) 1/14.9/14+ 5/14.1/14-15/14 d) B= 2/11.13 + 2/13.15 + 2/15.17+..+2/53.55

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1. Câu 1:

   a.-8/18-15/27=-4/9-5/9=-1.

   b.-3/2:9/4=-3/2.4/9=-2/3.

   c.1/14.9/14+5/14.1/14-15/14

     =1/14.9/14+5/14.1/14-15.1/14

     =1/14.(9/14+5/14-15)

     =1/14.(-14)

     =-1.

   d.B=2/11.13+2/13.15+2/15.17+…+2/53.55

         =1/11-1/13+1/13-1/15+1/15-1/17+…+1/53-1/55

         =1/11-1/55

         =4/55.

    

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2. a) -8/18-15/27=-1

   b)-3/2:9/4

   =-2/3

   c)1/14×9/14+5/14×1/14-15/14

   =1/14.(9/14+5/14)-15/14

   =1/14.1-15/14

   =1/14-15/14=-1

    

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