Đáp án:
a) $(2x - 3)(4 - x)$
$= 8x - 2x^{2} - 12 + 3x$
b) $\dfrac{3x + 5}{6x^{3}y} + \dfrac{15x - 5}{6x^{3}y}$
$= \dfrac{3x + 5 + 15x - 5}{6x^{3}y}$
$= \dfrac{(3x + 15x)+(5 - 5)}{6x^{3}y}$
$= \dfrac{18x}{6x^{3}y}$
$= \dfrac{3}{x^{2}y}$
c)$\dfrac{x}{x - 1} + \dfrac{1}{x + 1} + \dfrac{2 - 4x}{x^{2} - 1}$
$= \dfrac{x}{x - 1} + \dfrac{1}{x + 1} + \dfrac{2 - 4x}{(x - 1)(x + 1)} (MTC: (x - 1)(x + 1)$
$= \dfrac{x(x + 1)}{(x - 1)(x + 1)} + \dfrac{1(x - 1)}{(x - 1)(x + 1)} + \dfrac{2 - 4x}{(x - 1)(x + 1)}$
$= \dfrac{x^{2} + x + x - 1 + 2 - 4x}{(x - 1)(x + 1)}$
$= \dfrac{x^{2} + (x + x - 4x) + (-1 + 2)}{(x - 1)(x + 1)}$
$= \dfrac{x^{2} - 2x + 1}{(x - 1)(x + 1)}$
$= \dfrac{x - 1)^{2}}{(x - 1)(x + 1)}$
$= \dfrac{x - 1}{(x + 1)}$
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