Giải thích các bước giải:
g) $\frac{3}{4}$ + $\frac{x}{5}$ = $\frac{-9}{10}$
$\frac{3.5}{4.5}$ + $\frac{x.4}{5.4}$ = $\frac{(-9).2}{10.2}$
$\frac{15}{20}$ + $\frac{x.4}{20}$ = $\frac{-18}{20}$
$\frac{x.4}{20}$ = $\frac{-18}{20}$ - $\frac{15}{20}$
$\frac{x.4}{20}$ = $\frac{-33}{20}$
x . 4 = -33
x = -33 : 4
x = $\frac{-33}{4}$
e) $\frac{x}{3}$ + $\frac{x}{4}$ = $\frac{21}{42}$
x : 3 + x : 4 = $\frac{21}{42}$
x . $\frac{1}{3}$ + x . $\frac{1}{4}$ = $\frac{1}{2}$
x . ( $\frac{1}{3}$ + $\frac{1}{4}$ ) = $\frac{1}{2}$
x . $\frac{7}{12}$ = $\frac{1}{2}$
x = $\frac{1}{2}$ : $\frac{7}{12}$
x = $\frac{6}{7}$
