Tính theo cách hợp lí: a/ 4/20+16/42+ 6/15+ -3/5 +2/21+ -10/21+3/20 b/ 42/46+ 250/186+ -2121/2323+ -125125/143143

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1. Đáp án+Giải thích các bước giải:

    $\\a)\dfrac{4}{20}+\dfrac{16}{42}+\dfrac{6}{15}+\dfrac{-3}{5}+\dfrac{2}{21}+\dfrac{-10}{21}+\dfrac{3}{20}\\=\left(\dfrac{4}{20}+\dfrac{-3}{5}+\dfrac{6}{15}+\dfrac{3}{20}\right)+\left(\dfrac{16}{42}+\dfrac{2}{21}+\dfrac{-10}{21}\right)\\=\left(\dfrac{4}{20}+\dfrac{-12}{20}+\dfrac{8}{20}+\dfrac{3}{20}\right)+\left(\dfrac{8}{21}+\dfrac{2}{21}+\dfrac{-10}{21}\right)\\=\dfrac{3}{20}$ 

   $\\b)\dfrac{42}{46}+ \dfrac{250}{186}+ \dfrac{-2121}{2323}+ \dfrac{-125125}{143143}\\=\dfrac{21}{23}+ \dfrac{125}{93}+ \dfrac{-21}{23}+ \dfrac{-125}{143}\\=\left(\dfrac{21}{23}+\dfrac{-21}{23}\right)+\left(\dfrac{125}{93}+ \dfrac{-125}{143}\right)\\=\dfrac{6250}{13299}$

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