a) $\frac{1}{4}$ + $\frac{1}{3}$ : 2x = -5
$\frac{1}{4}$ + $\frac{1}{3}$.$\frac{1}{2}$x= -5
$\frac{1}{4}$ + $\frac{1}{6}$x = -5
$\frac{1}{6}$x = -5 - $\frac{1}{4}$
$\frac{1}{6}$x = $\frac{-21}{4}$
x = $\frac{-21}{4}$ : $\frac{1}{6}$
x = $\frac{63}{2}$
b) $\frac{11}{13}$ - ($\frac{5}{42}$ - x ) = -($\frac{15}{28}$ - $\frac{11}{3}$ )
$\frac{11}{13}$ - $\frac{5}{42}$ + x = -$\frac{15}{28}$ + $\frac{11}{3}$
$\frac{11}{13}$ - $\frac{5}{42}$ + x - $\frac{11}{3}$ = -$\frac{15}{28}$
- $\frac{5}{42}$ + x = -$\frac{15}{28}$
x = -$\frac{15}{28}$ + $\frac{5}{42}$
x = $\frac{45+10}{84}$
x = $\frac{55}{84}$
c) $\frac{-2}{5}$.(2 - $\frac{5}{6}$) - $\frac{2}{3}$. (-x +$\frac{1}{2}$) = $\frac{-3}{5}$
$\frac{-4}{5}$ + $\frac{1}{3}$x + $\frac{2}{3}$x - $\frac{2}{6}$ = $\frac{-3}{5}$
$\frac{34}{30}$ + x = $\frac{-3}{5}$
x = $\frac{-3}{5}$ - $\frac{17}{15}$
x = $\frac{-26}{15}$
d) (x + $\frac{3}{4}$)$^{2}$ = $\frac{4}{25}$
(x + $\frac{3}{4}$)$^{2}$ = ($\frac{2}{5}$)$^{2}$
=> x + $\frac{3}{4}$ = $\frac{2}{5}$
x = $\frac{2}{5}$ - $\frac{3}{4}$
x = $\frac{-7}{20}$
e) |15+x| +x = -15
|15+x| = -15 - x
|15+x| = -(15 + x)
=> x ∈ ∅
n*) 7x² -14x = 0
x( 7x -14)= 0
hoặc x = 0
hoặc 7x - 14= 0
7x = 0+14= 14
x = 14 : 7 = 2
Vậy x ∈ {0; 2}