BÀI 2B. Tính nhanh A) -5/9 + 8/15 + -2/11 + 4/-9 + 7/15 B) 4/45 + -1/15 + 7/45 + 4/15 + -2/45 + -1/5

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1. Đáp án+Giải thích các bước giải:

   $\\a)\dfrac{-5}{9}+\dfrac{8}{15}+\dfrac{-2}{11}+\dfrac{4}{-9}+\dfrac{7}{15}$ $\\=(\dfrac{-5}{9}+\dfrac{4}{-9})+\dfrac{-2}{11}+(\dfrac{8}{15}+\dfrac{7}{15})$ $\\=-1+\dfrac{-2}{11}+1$ $\\=\dfrac{-2}{11}+(-1+1)$ $\\=\dfrac{-2}{11}+0$ $\\=\dfrac{-2}{11}$

   $\\b)\dfrac{4}{45} + \dfrac{-1}{15} +\dfrac{7}{45} + \dfrac{4}{15} + \dfrac{-2}{45} + \dfrac{-1}{5}$ $\\=(\dfrac{4}{45} + \dfrac{7}{45} +\dfrac{-2}{45}) +( \dfrac{-1}{15} + \dfrac{4}{15}) + \dfrac{-1}{5}$ $\\=\dfrac{9}{45}+\dfrac{3}{15}+\dfrac{-1}{5}$$\\=\dfrac{1}{5}+\dfrac{1}{5}+\dfrac{-1}{5}$$\\=\dfrac{1}{5}$

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2. `#Kenshiro`

   `a) -5/9 + 8/15 + (-2/11) +(- 4/9) + 7/15`

   `= (-5/9 + -4/9) + (8/15 + 7/15) -2/11`

   `= -1 + 1 – 2/11`

   `= -2/11`

   `b ) 4/45 + -1/15 + 7/45 + 4/15 + -2/45 + -1/5`

   `= 4/45 + -3/45 + 7/45 + 12/45 +  -2/45 + -9/45`

   `= 1/5`

    

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