Đáp án:

 $VD5)
x=\dfrac{42}{115}\\
VD6:
x=\dfrac{-169}{1760}\\
VD7:
x=\dfrac{586}{261}\\
VD8:
x=\dfrac{-2}{5}\\$

Giải thích các bước giải:

 $VD5)
\dfrac{1}{3}-\dfrac{2}{7}x+\dfrac{4}{5}=\dfrac{2}{15}+3x-\dfrac{1}{5}\\
\Leftrightarrow 3x+\dfrac{2}{7}x=\dfrac{1}{3}+\dfrac{4}{5}-\dfrac{2}{15}+\dfrac{1}{5}\\
\Leftrightarrow \left (\dfrac{21}{7}+\dfrac{2}{7}  \right )x=\dfrac{1}{3}+\dfrac{5}{5}-\dfrac{2}{15}\\
\Leftrightarrow \dfrac{23}{7}x=\dfrac{5}{15}+\dfrac{15}{15}-\dfrac{2}{15}\\
\Leftrightarrow \dfrac{23}{7}x=\dfrac{18}{15}=\dfrac{6}{5}\\
\Leftrightarrow x=\dfrac{6}{5}.\dfrac{7}{23}=\dfrac{42}{115}\\
VD6:
\dfrac{3}{5}+\dfrac{6}{15}x-\dfrac{1}{25}+2x=-\dfrac{4}{13}x+\dfrac{3}{10}\\
\Leftrightarrow \dfrac{6}{15}x+2x+\dfrac{4}{13}x=\dfrac{3}{10}-\dfrac{3}{5}+\dfrac{1}{25}\\
\Leftrightarrow \left (\dfrac{2}{5}+2+\dfrac{4}{13}  \right )x=\dfrac{15}{50}-\dfrac{30}{50}+\dfrac{2}{50}\\
\Leftrightarrow \left (\dfrac{26}{65}+\dfrac{130}{65}+\dfrac{20}{65}  \right )x=\dfrac{-13}{50}\\
\Leftrightarrow \dfrac{176}{65}x=\dfrac{-13}{50}\\
\Leftrightarrow x=\dfrac{-13}{50}.\dfrac{65}{176}=\dfrac{-169}{1760}\\
VD7:
14-\left ( \dfrac{5}{7}x+\dfrac{1}{3} \right )=6x-\left ( \dfrac{2}{7}+\dfrac{1}{2}x \right )\\
\Leftrightarrow 14-\dfrac{5}{7}x-\dfrac{1}{3}=6x-\dfrac{2}{7}-\dfrac{1}{2}x\\
\Leftrightarrow \dfrac{-5}{7}x-6x+\dfrac{1}{2}x=\dfrac{-2}{7}-14+\dfrac{1}{3}\\
\Leftrightarrow \left (\dfrac{-5}{7}-6+\dfrac{1}{2}  \right )x=\dfrac{-6}{21}-\dfrac{294}{21}+\dfrac{7}{21}\\
\Leftrightarrow \left (\dfrac{-10}{14}-\dfrac{84}{14}+\dfrac{7}{14}  \right )x=\dfrac{-293}{21}\\
\Leftrightarrow \dfrac{-87}{14}x=\dfrac{-293}{21}\\
\Leftrightarrow x=\dfrac{-293}{21}.\dfrac{-14}{87}=\dfrac{586}{261}\\
VD8:
\dfrac{5}{4}+\left ( \dfrac{1}{2}x-\dfrac{11}{5}+\dfrac{3}{10} \right )=\dfrac{x}{8}-\dfrac{4}{5}\\
\Leftrightarrow \dfrac{1}{2}x-\dfrac{1}{8}x=\dfrac{-4}{5}-\dfrac{5}{4}+\dfrac{11}{5}-\dfrac{3}{10}\\
\Leftrightarrow \left (\dfrac{4}{8}-\dfrac{1}{8}  \right )x=\dfrac{-4+11}{5}-\dfrac{5}{4}-\dfrac{3}{10}\\
\Leftrightarrow \dfrac{3}{8}x=\dfrac{7}{5}-\dfrac{5}{4}-\dfrac{3}{10}\\
\Leftrightarrow \dfrac{3}{8}x=\dfrac{28}{20}-\dfrac{25}{20}-\dfrac{6}{20}\\
\Leftrightarrow \dfrac{3}{8}x=\dfrac{-3}{20}\\
\Leftrightarrow x=\dfrac{-3}{20}.\dfrac{8}{3}=\dfrac{-2}{5}\\$