Đáp án:

`A` `=` `-` $\dfrac{3}{20}$

Giải thích các bước giải:

  `A` `=` $\dfrac{-1}{20}$ `+` $\dfrac{-1}{30}$ `+` $\dfrac{-1}{42}$ `+` $\dfrac{-1}{56}$ `+` $\dfrac{-1}{72}$ `+` $\dfrac{-1}{90}$

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

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

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

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

   `A` `=` `-` $\dfrac{3}{20}$

 Vậy `A` `=` `-` $\dfrac{3}{20}$

Ta có :

`A = (-1)/20 + (-1)/30 + (-1)/42 + (-1)/56 + (-1)/72 + (-1)/90`

`=> A = -(1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90)`

`=> A = -(1/(4.5) + 1/(5.6) + 1/(6.7) + 1/(7.8) + 1/(8.9) + 1/(9.10))`

`=> A = -(1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10)`

`=> A = -(1/4 - 1/10)`

`=> A = -3/20`

Vậy `A = -3/20`

`#dtkc`