\(\frac{3x-1}{8}+\frac{3x+18}{11}=\frac{3x}{7}+\frac{3x+20}{13}\)
\(\Rightarrow\frac{1001\left(3x-1\right)}{8008}+\frac{728\left(3x+18\right)}{8008}=\frac{1144.3x}{8008}+\frac{616\left(3x+20\right)}{8008}\)
\(\Rightarrow3003x-1001+2184x+13104x=3432x+1848x+12320\)\
\(\Rightarrow\)\(19111x=13321\Rightarrow x=\frac{13321}{19111}\)
\(\frac{3\times-1}{8}+\frac{3\times+18}{11}=\frac{3\times}{7}+\frac{3\times+20}{13}\)
\(\Rightarrow\frac{3\times-1}{8}-\frac{3\times}{7}=\frac{3\times+20}{13}-\frac{3\times+18}{11}\)
\(\Rightarrow\frac{3\times-1}{8}+1-\frac{3\times}{7}-1=\frac{3\times+20}{13}-1-\frac{3\times+18}{11}+1\)
\(\Rightarrow\left(\frac{3\times-1}{8}+1\right)-\left(\frac{3\times}{7}+1\right)=\left(\frac{3\times+20}{13}-1\right)-\left(\frac{3\times+18}{11}-1\right)\)
\(\Rightarrow\frac{3\times+7}{8}-\frac{3\times+7}{7}=\frac{3\times+7}{13}-\frac{3\times+7}{11}\)
\(\Rightarrow\left(3\times+7\right)\cdot\left(\frac{1}{8}-\frac{1}{7}\right)=\left(3\times+7\right)\cdot\left(\frac{1}{13}-\frac{1}{11}\right)\)
\(\Rightarrow\left(3\times+7\right)\cdot\left(\frac{-1}{56}\right)=\left(3\times+7\right)\cdot\left(\frac{-2}{143}\right)\)
\(\Leftrightarrow3\times+7=0\)
\(\Leftrightarrow3\times=-7\)
\(\Leftrightarrow\times=\frac{-7}{3}\)
\(\frac{-7}{3}\inℚ\)
Vậy \(\times=\frac{-7}{3}\)