Bài 1 : tính giới hạn của
\(\lim\limits_{x\rightarrow1}\frac{4x^6-5x^5+x}{\left(1-x\right)^2}\)
Bài 2: chứng minh rằng
\(\sqrt{x^2+px+q}=\left|x+\frac{p}{2}\right|+\varepsilon\left(x\right)\) với \(\lim\limits_{x\rightarrow+\infty}\varepsilon\left(x\right)=0\)
Bài 3: tìm a và b sao cho
\(\lim\limits_{x\rightarrow+\infty}\left[\sqrt{9x^2-4x+3}-\left(ax+b\right)\right]=0\)
Bài 1:
\(\lim\limits _{x\to 1}\frac{4x^6-5x^5+x}{(1-x)^2}=\lim\limits _{x\to 1}\frac{x(x-1)^2(4x^3+3x^2+2x+1)}{(1-x)^2}\)
\(=\lim\limits _{x\to 1}x(4x^3+3x^2+2x+1)=1(4.1^3+3.1^2+2.1+1)=10\)
Bài 3:
\(\lim\limits _{x\to +\infty}[\sqrt{9x^2-4x+3}-(ax+b)]=0\)
\(\Rightarrow \lim\limits _{x\to +\infty}\frac{\sqrt{9x^2-4x+3}-(ax+b)}{x}=0\)
\(\Leftrightarrow \lim\limits _{x\to +\infty}\left(\sqrt{9-\frac{4}{x}+\frac{3}{x^2}}-a+\frac{b}{x}\right)=0\)
\(\Leftrightarrow a=3\)
Thay $a=3$ vào đk ban đầu:
\(\lim\limits _{x\to +\infty}[\sqrt{9x^2-4x+3}-3x-b]=0\)
\(\Leftrightarrow \lim\limits _{x\to +\infty} (\sqrt{9x^2-4x+3}-3x)=b\)
\(\Leftrightarrow \lim\limits _{x\to +\infty}\frac{-4x+3}{\sqrt{9x^2-4x+3}+3x}=b\)
\(\Leftrightarrow \lim\limits _{x\to +\infty}\frac{-4+\frac{3}{x}}{\sqrt{9-\frac{4}{x}+\frac{3}{x}}+3}=b\)
\(\Leftrightarrow \frac{-4}{6}=b\Leftrightarrow b=-\frac{2}{3}\)
Bài 2:
\(\lim\limits _{x\to +\infty}\varepsilon(x)=\lim\limits _{x\to +\infty}[\sqrt{x^2+px+q}-|x+\frac{p}{2}|]=\lim\limits _{x\to +\infty}\frac{x^2+px+q-(x^2+px+\frac{p^2}{4})}{\sqrt{x^2+px+q}+|x+\frac{p}{2}|}\)
\(=\lim\limits _{x\to +\infty}\frac{q-\frac{p^2}{4}}{\sqrt{x^2+px+q}+|x+\frac{p}{2}|}=0\) do \(\sqrt{x^2+px+q}+|x+\frac{p}{2}|\to +\infty \)
\(\Rightarrow \sqrt{x^2+px+q}=|x+\frac{p}{2}|+\varepsilon (x)\) với \(\lim\limits _{x\to +\infty}\varepsilon (x)=0\)