Lời giải

Đáp án đúng là: D

$\frac{{x + 3y}}{{4x + 8y}}\,\,.\,\,A = \frac{{{x^2} - 9{y^2}}}{{x + 2y}}$

$A = \frac{{{x^2} - 9{y^2}}}{{x + 2y}}:\frac{{x + 3y}}{{4x + 8y}}$

$= \frac{{\left( {x - 3y} \right)\left( {x + 3y} \right)}}{{x + 2y}}:\frac{{x + 3y}}{{4\left( {x + 2y} \right)}}$

$= \frac{{\left( {x - 3y} \right)\left( {x + 3y} \right)}}{{x + 2y}} \cdot \frac{{4\left( {x + 2y} \right)}}{{x + 3y}} = 4\left( {x - 3y} \right)$.