Đáp án:
$a)x=\dfrac{-15}{4}\\
b) x=\dfrac{-1}{4}\\
c) x=\dfrac{9}{73}\\
d) x=-\dfrac{5}{11}$
Giải thích các bước giải:
$a) \dfrac{7}{8}+4\dfrac{1}{2}:x=-\dfrac{13}{40}\\
\Leftrightarrow \dfrac{9}{2}.\dfrac{1}{x}=-\dfrac{13}{40}-\dfrac{7}{8}\\
\Leftrightarrow \dfrac{9}{2}.\dfrac{1}{x}=-\dfrac{13}{40}-\dfrac{35}{40}\\
\Leftrightarrow \dfrac{9}{2}.\dfrac{1}{x}=-\dfrac{48}{40}=\dfrac{-6}{5}\\
\Leftrightarrow x=\dfrac{9}{2}.\dfrac{-5}{6}=\dfrac{-15}{4}\\
b) (x-2\dfrac{1}{4}):(-\dfrac{5}{6})=3\\
\Leftrightarrow (x-\dfrac{9}{4}).(-\dfrac{6}{5})=3\\
\Leftrightarrow x-\dfrac{9}{4}=3.\dfrac{-5}{6}\\
\Leftrightarrow x=\dfrac{-5}{2}+\dfrac{9}{4}\\
\Leftrightarrow x=\dfrac{-10}{4}+\dfrac{9}{4}\\
\Leftrightarrow x=\dfrac{-1}{4}\\
c) 4\dfrac{2}{3}-(\dfrac{3}{5}):x=-20.\dfrac{1}{100}\\
\Leftrightarrow \dfrac{14}{3}-\dfrac{3}{5}.\dfrac{1}{x}=-\dfrac{1}{5}\\
\Leftrightarrow -\dfrac{3}{5}\dfrac{1}{x}=\dfrac{-1}{5}-\dfrac{14}{3}\\
\Leftrightarrow -\dfrac{3}{5}\dfrac{1}{x}=\dfrac{-3}{15}-\dfrac{14.5}{15}\\
\Leftrightarrow -\dfrac{3}{5}\dfrac{1}{x}=\dfrac{-73}{15}\\
\Leftrightarrow x=\dfrac{-3}{5}.\dfrac{-15}{73}=\dfrac{9}{73}\\
d) \dfrac{70}{100}x=1\dfrac{1}{2}+2.2x\\
\Leftrightarrow \dfrac{7}{10}x=\dfrac{3}{2}+4x\\
\Leftrightarrow \left (\dfrac{7}{10}-4 \right )x=\dfrac{3}{2}\\
\Leftrightarrow \left (\dfrac{7}{10}-\dfrac{4.10}{10} \right )x=\dfrac{3}{2}\\
\Leftrightarrow \dfrac{-33}{10}x=\dfrac{3}{2}\\
\Leftrightarrow x=\dfrac{3}{2}.\dfrac{-10}{33}=-\dfrac{5}{11}$
