Câu 1.27.
a. x - $\left ( \frac{5}{4}-\frac{7}{5} \right )$= $\frac{9}{20}$
=> x = $\frac{9}{20}$ + $\left ( \frac{5}{4}-\frac{7}{5} \right )$ => x= $\frac{9}{20}$ + $\left ( \frac{25}{20}-\frac{28}{20} \right )$ = > x = $\frac{9}{20}$ + $\frac{-3}{20}$ => x= $\frac{6}{20}$=$\frac{3}{10}$
b. 9 - x = $\frac{8}{7}$ - (-$\frac{7}{8}$)
=> 9 - x = $\frac{8}{7}$ + $\frac{7}{8}$ => 9 - x = $\frac{64}{56}$ + $\frac{49}{56}$ = > 9 - x = $\frac{113}{56}$ => 9 - $\frac{113}{56}$ = x=> x= $\frac{504}{56}$ - $\frac{113}{56}$=> x= $\frac{391}{56}$