Lời giải:
$\frac{11}{13}-(\frac{5}{42}-x)=-(\frac{15}{28}-\frac{11}{13}=\frac{11}{13}-\frac{15}{28}$
$\Rightarrow \frac{5}{42}-x=\frac{15}{28}$
$x=\frac{5}{42}-\frac{15}{28}=\frac{-5}{12}$
\(\dfrac{11}{13}-\left(\dfrac{5}{42}-x\right)=-\left(\dfrac{15}{28}-\dfrac{11}{13}\right)\)
\(\Rightarrow\dfrac{11}{13}-\left(\dfrac{5}{42}-x\right)=-\dfrac{15}{28}+\dfrac{11}{13}\)
\(\Rightarrow-\dfrac{5}{42}+x=-\dfrac{15}{28}+\dfrac{11}{13}-\dfrac{11}{13}\)
\(\Rightarrow-\dfrac{5}{42}+x=-\dfrac{15}{28}\)
\(\Rightarrow x=-\dfrac{15}{28}+\dfrac{5}{42}\)
\(\Rightarrow x=-\dfrac{5}{12}\)
