\(\frac{7}{13}.\left(\frac{7}{12}\right)-\frac{5}{12}.\left(\frac{21}{39}\right)+\frac{49}{91}.\left(\frac{8}{5}\right)=\frac{7}{13}.\left(\frac{7}{12}\right)-\frac{5}{12}.\left(\frac{7}{13}\right)+\frac{7}{13}.\left(\frac{8}{5}\right)=\frac{7}{13}.\left(\frac{7}{15}-\frac{5}{12}+\frac{8}{5}\right)=\frac{7}{13}.\left(\frac{33}{20}\right)=\frac{261}{260}\)

buồi

\(\frac{7}{8}=x-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-...-\frac{1}{120}\)

\(\frac{7}{4}=2x-\left(\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\right)\)

\(\frac{7}{4}=2x-2\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\right)\)

\(\frac{7}{4}=2x-2\left(\frac{1}{4}-\frac{1}{16}\right)\)

\(\frac{7}{4}=2x-\frac{3}{8}\)

\(2x=\frac{17}{8}\)

\(x=\frac{17}{16}\)

Vậy \(x=\frac{17}{16}\)

\(=15,7\times1+15,7\times1+15,7\times1+15,7\times8-15,7\times1\\ =15,7\times\left(1+1+1+8-1\right)\\ 15,7\times10=157\)

`15,7+15,7+15,7+15,7xx8-15,7`

`=15,7xx3+15,7xx8-15,7xx1`

`=15,7xx(3+8-1)`

`=15,7xx10`

`=157`

`@An`

kết quả là D nhé

Học tốt nhen

D nhé bn ui

Bài này tự làm

\(\dfrac{7}{15}+\dfrac{4}{5}=\dfrac{7}{15}+\dfrac{12}{15}=\dfrac{19}{15}\)

\(\dfrac{2}{3}-\dfrac{3}{8}=\dfrac{16}{24}-\dfrac{9}{24}=\dfrac{7}{24}\)

\(\dfrac{4}{7}.\dfrac{21}{16}:\dfrac{5}{8}=\dfrac{4}{7}.\dfrac{21}{16}.\dfrac{8}{5}=\dfrac{4.3.7.4.2}{7.4.4.5}=\dfrac{6}{5}\)

giup minh voi minh can gap lam

\(A=\frac{4}{3.7}+\frac{4}{7.11}+...+\frac{4}{107.111}\)

\(A=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{107}-\frac{1}{111}\)

\(A=\frac{1}{3}-\frac{1}{111}\)

\(A=\frac{12}{37}\)

mà dài quá bạn ơi ban tách ra thành nhiều câu hỏi đi thế này trả lời lâu lắm