Tìm x, biết: a) x + 3/5 = 2/3 b) 3/7 - x = 2/5; c) 4/9 - 2/3x

* Hướng dẫn giải

a) $x+\frac{3}{5}=\frac{2}{3}$

$$\begin{gathered} x=\frac{2}{3}-\frac{3}{5} \\ x=\frac{10}{15}-\frac{9}{15} \\ x=\frac{1}{15} \end{gathered}$$

Vậy $x=\frac{1}{15}$

b) $\frac{3}{7}-x=\frac{2}{5}$

$$\begin{gathered} x=\frac{3}{7}-\frac{2}{5} \\ x=\frac{15}{35}-\frac{14}{35} \\ x=\frac{1}{35} \end{gathered}$$

Vậy $x=\frac{1}{35}$

c) $\frac{4}{9}-\frac{2}{3}x=\frac{1}{3}$

$$\begin{gathered} \frac{2}{3}x=\frac{4}{9}-\frac{1}{3} \\ \frac{2}{3}x=\frac{4}{9}-\frac{3}{9} \\ \frac{2}{3}x=\frac{1}{9} \\ x=\frac{1}{9}:\frac{2}{3} \\ x=\frac{1}{9}.\frac{3}{2} \\ x=\frac{1}{6} \end{gathered}$$

Vậy $x=\frac{1}{6}$

d) $\frac{3}{10}x-1\frac{1}{2}=\left(\frac{-2}{7}\right):\frac{5}{14}$

$$\begin{gathered} \frac{3}{10}x-\frac{3}{2}=\frac{-2}{7}.\frac{14}{5} \\ \frac{3}{10}x-\frac{3}{2}=\frac{-4}{5} \\ \frac{3}{10}x=\frac{-4}{5}+\frac{3}{2} \\ \frac{3}{10}x=\frac{-8}{10}+\frac{15}{10} \\ \frac{3}{10}x=\frac{7}{10} \\ x=\frac{7}{10}:\frac{3}{10} \\ x=\frac{7}{10}.\frac{10}{3} \\ x=\frac{7}{3} \end{gathered}$$

Vậy$x=\frac{7}{3}$