Giải thích các bước giải:
a.$\dfrac{80x^3-125x}{3(x-3)-(x-3)(8-4x)}$
$=\dfrac{5x(16x^2-25)}{(x-3)(3-8+4x)}$
$=\dfrac{5x(4x-5)(4x+5)}{(x-3)(4x-5)}$
$=\dfrac{5x(4x-5)}{x-3}$
b.$\dfrac{9-(x+5)^2}{x^2+4x+4}$
$=\dfrac{(3-(x+5))(3+(x+5))}{(x+2)^2}$
$=\dfrac{-(x+2)(x+8)}{(x+2)^2}$
$=\dfrac{-(x+8)}{x+2}$