Giải bài 4 trang 25

Đề bài

Tìm x, biết:

a) $x+\frac{3}{5}=\frac{2}{3};$                      

b) $\frac{3}{7}-x=\frac{2}{5};$

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

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

Lời giải chi tiết

a) $x+\frac{3}{5}=\frac{2}{3}$

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

Vậy $x=\frac{1}{15}$.

b) $\frac{3}{7}-x=\frac{2}{5}$

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

Vậy $x=\frac{1}{35}$.

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

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

Vậy $x=\frac{1}{6}$.

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

$\begin{array}{c}\frac{3}{10}x-\frac{3}{2}=\left(\frac{-2}{7}\right).\frac{14}{5}\\ \frac{3}{10}x-\frac{3}{2}=\frac{-4}{5}\\ \frac{3}{10}x=\frac{-4}{5}+\frac{3}{2}\\ \frac{3}{10}x=\frac{-8}{10}+\frac{15}{10}\\ \frac{3}{10}x=\frac{7}{10}\\ x=\frac{7}{10}:\frac{3}{10}\\ x=\frac{7}{3}\end{array}$

Vậy $x=\frac{7}{3}$.