Đáp án:
Giải thích các bước giải:
1)
x = $\dfrac{1}{4}$ + $\dfrac{2}{13}$
x = $\dfrac{21}{52}$
2)
$\dfrac{x}{3-x}$ = $\dfrac{-5}{21}$
$21.x = (3-x)-5$
$21x = - 15 + 5x$
$21x - 5x = - 15$
$- 16x = -15$
x = $\dfrac{15}{16}$
3)
$\dfrac{3}{7-x}$ = $\dfrac{-5}{21}$
$3.21=(7-x)(-5)$
$63=-35 + 5x$
$63+35 = 5x$
$98=5x$
x=$\dfrac{98}{5}$