Đáp án:
$A=\dfrac{227}{299}\\
b)x=7$
Giải thích các bước giải:
$A=-\dfrac{11}{23}.\dfrac{10}{-13}+\dfrac{-3}{23}-\left ( -\dfrac{12}{23} \right )\\
=\dfrac{11}{23}.\dfrac{10}{13}-\dfrac{3}{23}+\dfrac{12}{23}\\
=\dfrac{110}{23.13}+\dfrac{-3+12}{23}\\
=\dfrac{110}{23.13}+\dfrac{9}{23}\\
=\dfrac{1}{23}\left ( \dfrac{110}{13}+9 \right )\\
=\dfrac{1}{23}. \dfrac{110+117}{13}\\
=\dfrac{227}{299}\\
b)\dfrac{x-1}{4}=-\dfrac{9}{1-x}\\
\Leftrightarrow \dfrac{x-1}{4}=\dfrac{9}{x-1}\\
\Leftrightarrow (x-1)^2=9.4\\
\Leftrightarrow (x-1)^2=36\\
\Leftrightarrow x-1=6\\
\Leftrightarrow x=7$
