Bài 1:
$B=\dfrac{\dfrac{1}{3}-\dfrac{4}{5}}{\dfrac{1}{3}-\dfrac{4}{5}}.\dfrac{\dfrac{3}{4}-\dfrac{5}{3}}{\dfrac{3}{4}+\dfrac{5}{3}}.\dfrac{\dfrac{4}{5}-1}{1-\dfrac{2}{3}}\\B=1.\dfrac{-\dfrac{11}{12}}{\dfrac{29}{12}}:\dfrac{-\dfrac{1}{5}}{\dfrac{1}{3}}\\B=\dfrac{-11}{29}.\frac{\dfrac{1}{3}}{-\dfrac{1}{5}}\\B=\dfrac{11}{29}.\dfrac{5}{3}=\dfrac{55}{87}$
Bài 2:
$b)3^{-1}.3^{n}+6.3^{n-1}=7.3^{6}\\\Leftrightarrow 3^{n-1}+6.3^{n-1}=7.3^{6}\\\Leftrightarrow 7.3^{n-1}=7.3^{6}\\\Leftrightarrow 3^{n-1}=3^{6}\\\Leftrightarrow n-1=6\\\Leftrightarrow n=6+1=7$
Vậy $n=7$
