D

211,6

4/28 + 4/168 + 4/408 + 4/748 + 4/1188 + 4/1728

= 1/7 + 1/42 + 1/102 + 1/187 + 1/297 + 1/432

= 1/7 + 1/7.6 + 1/6.17 + 1/17.11 + 1/11.27 + 1/27.16

= 1/7(1 + 1/6) + 1/17(1/6 + 1/11) + 1/27(1/11 + 1/16)

= 1/7.7/6 + 1/17.17/66 + 1/27.27/176

= 1/6 + 1/66 + 1/176

= 12/66 + 1/176

= 3/16

a=3\12

b=5\4 

`Answer:`

a. \(-\frac{13}{31}:\frac{5}{7}+\left(-\frac{18}{31}\right):\frac{5}{7}=\left(-\frac{13}{31}+-\frac{18}{31}\right):\frac{5}{7}=-1:\frac{5}{7}=-\frac{7}{5}\)

b. \(0,5+\frac{1}{3}+0,4+\frac{5}{7}+\frac{1}{6}-\frac{4}{35}\)

\(=\frac{1}{2}+\frac{1}{3}+\frac{2}{5}+\frac{5}{7}+\frac{1}{6}-\frac{4}{35}\)

\(=\left(\frac{1}{2}+\frac{1}{3}+\frac{1}{6}\right)+\left(\frac{2}{5}+\frac{5}{7}-\frac{4}{35}\right)\)

\(=1+1\)

\(=2\)

c. \(-1.2.3.4...9+1.2.3...8+1.2.3...8^2\)

\(=-1.2.3.4...9+1.2.3...8.\left(1+8\right)\)

\(=-1.2.3.4...9+1.2.3...8.9\)

\(=0\)

\(a)\frac{-2}{3}\cdot\frac{5}{17}+\frac{-2}{3}\cdot\frac{12}{17}+\frac{2}{3}\)

\(=\frac{-2}{3}\cdot\frac{5}{17}+\frac{-2}{3}\cdot\frac{12}{17}+\frac{-2}{3}\cdot\left(-1\right)\)

\(=\frac{-2}{3}\cdot\left[\frac{5}{17}+\frac{12}{17}+\left(-1\right)\right]\)

\(=\frac{-2}{3}\cdot0\)

\(=0\)

\(b)\frac{3}{4}\cdot\frac{9}{13}+\frac{3}{4}\cdot\frac{7}{13}-\frac{3}{14}\cdot\frac{3}{13}\)

\(=\frac{3}{4}\cdot\left(\frac{9}{13}+\frac{7}{13}-\frac{3}{13}\right)\)

\(=\frac{3}{4}\cdot1\)

\(=\frac{3}{4}\)

\(c)\frac{-7}{12}\cdot\frac{5}{11}+\frac{-7}{12}\cdot\frac{5}{11}-\frac{1}{3}\)

\(=\frac{-7}{12}\cdot\left(\frac{5}{11}+\frac{5}{11}\right)-\frac{1}{3}\)

\(=\frac{-7}{12}\cdot\frac{10}{11}-\frac{1}{3}\)

\(=\frac{-35}{66}-\frac{1}{3}\)

\(=\frac{-57}{66}\)

\(d)\frac{-1}{5}\cdot\frac{4}{7}+\frac{-1}{5}\cdot1\frac{3}{7}+\left(\frac{-2}{5}\right)^1\)\

\(=\frac{-1}{5}\cdot\frac{4}{7}+\frac{-1}{5}\cdot\frac{10}{7}+\frac{-2}{5}\)

\(=\frac{-1}{5}\cdot\frac{4}{7}+\frac{-1}{5}\cdot\frac{10}{7}+\frac{-1}{5}\cdot2\)

\(=\frac{-1}{5}\cdot\left(\frac{4}{7}+\frac{10}{7}+2\right)\)

\(=\frac{-1}{5}\cdot4\)

\(=\frac{-4}{5}\)

ủa sao ko ai trả lời v ạ 

em  mà hc lớp 6 thì em  đã giúp 

em mới hc lớp 5

`Answer:`

1.

\(A=\frac{1}{4}+\frac{2}{3}=\frac{3}{12}+\frac{8}{12}=\frac{11}{12}\)

\(B=\frac{3}{5}.\frac{5}{4}-\frac{3}{5}.\frac{1}{4}=\frac{3}{5}.\left(\frac{5}{4}-\frac{1}{4}\right)=\frac{3}{5}\)

2.

\(x-\frac{7}{13}=-\frac{21}{13}\)

\(\Leftrightarrow x=\left(-\frac{21}{13}\right)-\frac{7}{13}\)

\(\Leftrightarrow x=-\frac{14}{13}\)

\(\frac{1}{4}x-6\frac{2}{5}=3,75\)

\(\Leftrightarrow0,25x-6,4=3,75\)

\(\Leftrightarrow0,25x=3,75+6,4\)

\(\Leftrightarrow0,25x=10,15\)

\(\Leftrightarrow x=10,15:0,25\)

\(\Leftrightarrow x=40,6\)