A, n=1

B, n=1/4

C, n=5/3

-6x=-3x-15

-3x=-15

x=3

cau b:triet. het bang 1/7

\(\dfrac{1}{3}+\left(\dfrac{-1}{3.7}+\dfrac{-3.4}{3.7}\right)+\dfrac{5}{7}+\dfrac{1}{3.2}+\dfrac{1}{2.7}+\dfrac{-1}{2}=\dfrac{1}{3}-\dfrac{1}{3}.\dfrac{1}{7}-\dfrac{4}{7}+\dfrac{5}{7}+\dfrac{1}{3}.\dfrac{1}{2}+\dfrac{1}{2}.\dfrac{1}{7}-\dfrac{1}{2}=\left[\dfrac{1}{3}.\dfrac{6}{7}+\dfrac{1}{7}.\dfrac{3}{2}-\dfrac{1}{3}\right]=\dfrac{1}{3}.\dfrac{-1}{7}+\dfrac{1}{7}.\dfrac{3}{2}=\dfrac{1}{7}\left\{\dfrac{-2}{6}+\dfrac{9}{6}\right\}.=\dfrac{1}{6}\)