Đáp án:
câu 3
a) $\frac{4}{5}$-x=$\frac{1}{3}$ ->x=$\frac{4}{5}$- $\frac{1}{3}$= $\frac{7}{15}$
b) $\frac{3}{4}$+ $\frac{1}{4}$:x=$\frac{2}{5}$-> $\frac{1}{4}$:x= $\frac{2}{5}$- $\frac{3}{4}$= $\frac{-7}{20}$=>x=$\frac{1}{4}$: $\frac{-7}{20}$= $\frac{-5}{7}$