Câu 19: B
Câu 20: A
Câu 21: C
19.
\(\lim\limits_{x\rightarrow-\infty}2x^3-x^2=\lim\limits_{x\rightarrow-\infty}x^3\left(2-\dfrac{1}{x}\right)=-\infty.2=-\infty\)
20.
\(\lim\limits_{x\rightarrow+\infty}\left(\sqrt{x^2+1}-x\right)=\lim\limits_{x\rightarrow+\infty}\dfrac{1}{\sqrt{x^2+1}+x}=\lim\limits_{x\rightarrow+\infty}\dfrac{\dfrac{1}{x}}{\sqrt{1+\dfrac{1}{x^2}}+1}=\dfrac{0}{2}=0\)
21.
\(\lim\limits_{x\rightarrow2}\dfrac{x^3-8}{x^2-4}=\lim\limits_{x\rightarrow2}\dfrac{\left(x-2\right)\left(x^2+2x+4\right)}{\left(x-2\right)\left(x+2\right)}=\lim\limits_{x\rightarrow2}\dfrac{x^2+2x+4}{x+2}=\dfrac{12}{4}=3\)
