`A=`$\frac{-7}{21}+1+\frac{1}{3}$

$=\frac{-7}{21}+\frac{4}{3}$

$=1$

`B=`$\frac{2}{15}+\left(\frac{55}{9}+\frac{-6}{9}\right)$

$=\frac{2}{15}+\frac{55}{9}+\frac{-6}{9}$

$=\frac{2}{15}+\frac{55}{9}+\frac{-6}{9}$

$=\frac{6+245}{45}$

$=\frac{251}{45}$

`C=`$\left(\frac{-1}{5}+\frac{3}{12}\right)+\frac{-3}{4}$

$=\frac{-1}{5}+\frac{3}{12}+\frac{-3}{4}$

$=-\frac{1}{5}+\frac{1}{4}-\frac{3}{4}$

$=-\frac{1}{5}+\frac{-2}{4}$

$-\frac{2}{10}-\frac{5}{10}$

$=\frac{-7}{10}$$=-\frac{7}{10}$

`D=`$\frac{4}{20}+\frac{16}{42}+\frac{6}{15}+\frac{-3}{5}+\frac{2}{21}+\frac{-10}{21}+\frac{3}{20}$

$=\frac{4}{20}+\frac{16}{42}+\frac{6}{15}-\frac{3}{5}+\frac{2}{21}-\frac{10}{21}+\frac{3}{20}$

$=\frac{7}{20}+\frac{16}{42}+\frac{6}{15}-\frac{3}{5}+\frac{2}{21}-\frac{10}{21}$

$=\frac{7}{20}+\frac{16}{42}+\frac{6}{15}-\frac{3}{5}+\frac{-8}{21}$

$=\frac{7}{20}+\frac{8}{21}+\frac{2}{5}-\frac{3}{5}+\frac{-8}{21}$

$=\frac{7}{20}+\frac{0}{21}+\frac{-1}{5}$

$=\frac{147}{420}+\frac{0}{420}+\frac{-84}{420}$

$=\frac{63}{420}$$=\frac{3}{20}$

  Ron

A = `-7/21` + (1 + `1/3`)

    = `-7/21` + (1 + `7/21`)

    = `-7/21` + `28/21`

    = 1

B = `2/15` + (`5/9` + `-6/9`)

    = `2/15` + `5/9` + `-6/9`

    = `6/45` + `25/45` + `-30/45`

    = $\dfrac{6 + 25 - 30}{45}$

    = $\dfrac{1}{45}$

C = (`-1/5` + `3/12`) + `-3/4`

    = `-1/5` + `3/12` + `-3/4`

    = `-12/60` + `15/60` + `-45/60`

    = $\dfrac{-12 + 15 - 45}{60}$

    = $\dfrac{-42}{60}$

    = `-7/10`

D = `4/20` + `16/42` + `6/15` + `-3/5` + `2/21` + `-10/21` + `3/20`

    = `4/20` + `8/21` + `2/5` + `-3/5` + `2/21` + `-10/21` + `3/20`

    = (`4/20` + `3/20`) + (`8/21` + `2/21` + `-10/21`) + (`2/5` + `-3/5`)

    = `7/20` + 0 + `-1/5`

    = `7/20` + `-4/20`

    = $\dfrac{7 - 4}{20}$

    = `3/20`

      #Chúc bạn học tốt

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