tính a) -81/26×13/9 b) 1/5.7 + 1/7.9 +…+ 1/15.17

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1. a `-81/26 ×13/9= -9/2`

   b `1/5.7 +1/7.9 +…+ 1/15.17`

   = `1/2.(2/5.7 +2/7.9 +…+ 2/15.17)`

   =`1/2.(1/5.7+ 1/7.9+…+1/15.17)`

   = `1/2.(1/5 -1/7 +1/7 -1/9 +1/9+…-1/15 +1/15-1/17)`

   =`1/2.(1/5-1/17)`

   = `1/2.(17/85-5/85)`

   =`1/2. 12/85`

   =`6/85`

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2. $a$) $-81/26.13/9$

   $= -81.13/26.9$

   $= -9.1/2.1$

   $= -9/2$

   $b$) $1/5.7 + 1/7.9 +…+ 1/15.17$

   $= 1/2 . (2/5.7 + 2/7.9 +…+ 2/15.17)$

   $= 1/2 . (1/5 – 1/7 + 1/7 – 1/9 + … + 1/15 – 1/17)$

   $= 1/2 . (1/5 – 1/17)$

   $= 1/2 . 12/85$

   $= 6/85$

    

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