15 $\lim_{x\to -\infty}\dfrac{-\sqrt[]{4x^2-3x+4}-2x }{-\sqrt[]{x^2+x+1}-x }$
= $\lim_{x\to -\infty} \dfrac{-\sqrt[]{4-\dfrac{3}{x}+\dfrac{4}{x^2}}-2 }{-\sqrt[]{1+\dfrac{1}{x}+\dfrac{1}{x^2}}-1 }$
=$\dfrac{-2-2}{-1-1}$ =2=>C
16 $\lim_{x\to 3^+} \dfrac{|(x-5)(x-3)|}{x(x-3)}$
= $\lim_{x \to3^+} \dfrac{|x-5|}{x}$ =$\dfrac{2}{3}$ =>A
17 $\lim_{x \to (-1)^-}\dfrac{(x^2+3x+2)}{-(x+1)}$
= $\lim_{x\to (-1)^-} \dfrac{(x+1)(x+2)}{-(x+1)}$
= $\lim_{x \to (-1)^-}\dfrac{x+2}{-1}$ =-1=>C
hay thì xin hay nhất
