-1/20 + -1/30 + -1/42 + -1/56 +-1/72 + -1/90

0 bình luận về “-1/20 + -1/30 + -1/42 + -1/56 +-1/72 + -1/90”

1. $\dfrac{-1}{20} +  \dfrac{-1}{30} +\dfrac{-1}{42} + \dfrac{-1}{56} +\dfrac{-1}{72}+ \dfrac{-1}{90}$

   $= \dfrac{-1}{20} –  \dfrac{1}{30} – \dfrac{1}{42} – \dfrac{1}{56} – \dfrac{1}{72} –  \dfrac{1}{90}$

   $= – ( \dfrac{1}{20} + \dfrac{1}{30}  + \dfrac{1}{42}+ \dfrac{1}{56} +\dfrac{1}{72} + \dfrac{1}{90})$

   $= – (\dfrac{1}{4.5} + \dfrac{1}{5.6} + \dfrac{1}{6.7} + \dfrac{1}{7.8} + \dfrac{1}{8.9} + \dfrac{1}{9.10})$

   $= – ( \dfrac{1}{4} – \dfrac{1}{5} + \dfrac{1}{5} -\dfrac{1}{6} + \dfrac{1}{6} – \dfrac{1}{7} + \dfrac{1}{7} – \dfrac{1}{8} + \dfrac{1}{8} – \dfrac{1}{9} + \dfrac{1}{9}  – \dfrac{1}{10})$

   $= -(\dfrac{1}{4} – \dfrac{1}{10})$

   $= – \dfrac{3}{20}$

    

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2.  `Đáp` `án`

   `-1/20` `+` `-1/30` `+` `-1/42“+` `-1/56` `+` `-1/72“+` `-1/90`
   `=` `-` `(1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90)` 
   `=` `-` `(1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/ 9.10)`
   `=` `-“(1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)`
   `=` `-“(1/4+0+0+0+0+0-1/10)`
   `=` `-“(1/4-1/10)`
   `=“-“(10/40-4/40)`
   `=` `-3/20`

    

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