Bạn tham khảo:
a, $\dfrac{4}{5} = \dfrac{-12}{9-x}$
$⇔ 4( 9 - x ) = 4 . ( -12 )$
$⇔ 36 - 4x = -48$
$⇔ -4x = -48 - 36$
$⇔ -4x = -84$
$⇔ x = 21$
b, $\dfrac{x+1}{3} = \dfrac{3}{x+1}$
$⇔ ( x + 1 ) ² = 9$
$⇔ ( x + 1 )² - 9 = 0$
$⇔ ( x + 1 - 3 ) ( x + 1 + 3 ) = 0$
$⇔ ( x - 2 ) ( x + 4 ) = 0$
⇔ \(\left[ \begin{array}{l}x-2=0\\x+4=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=2\\x=-4\end{array} \right.\)
c, $\dfrac{x-1}{-4} = \dfrac{-4}{x-1}$
$⇔ ( x - 1 )² = 16 $
$⇔ ( x -1 )² - 16 = 0$
$⇔ ( x - 1 + 4 )(x -1 - 4 ) = 0$
$⇔ ( x + 3 )(x - 5 ) = 0$
⇔ \(\left[ \begin{array}{l}x+3=0\\x-5=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-3\\x=5\end{array} \right.\)
d, $\dfrac{x}{-5} = \dfrac{-5}{x}$
$⇔ x² = 25$
$⇔ x = ±5$
