Giải câu 5 trang 12 toán VNEN 7 tập 1

a) x + $\frac{1}{3}$ = $\frac{3}{4}$ $\Rightarrow$ x = $\frac{3}{4}$ -  $\frac{1}{3}$ $\Rightarrow$ x = $\frac{9}{12}$ - $\frac{4}{12}$ $\Rightarrow$ x = $\frac{5}{12}$;               

b) x - $\frac{2}{5}$ = $\frac{5}{7}$ $\Rightarrow$ x = $\frac{2}{5}$ + $\frac{5}{7}$ $\Rightarrow$ x = $\frac{14}{35}$ + $\frac{25}{35}$ $\Rightarrow$ x = $\frac{39}{35}$;        

c) -x - $\frac{2}{3}$ = -$\frac{6}{7}$ $\Rightarrow$ x =$\frac{6}{7}$ - $\frac{2}{3}$ $\Rightarrow$ x = $\frac{18}{21}$ - $\frac{14}{21}$ $\Rightarrow$ x = $\frac{4}{21}$; 

d) $\frac{4}{7}$ - x = $\frac{1}{3}$ $\Rightarrow$ x = $\frac{4}{7}$ - $\frac{1}{3}$ $\Rightarrow$ x = $\frac{12}{21}$ - $\frac{7}{21}$ $\Rightarrow$ x = $\frac{5}{21}$.