\( C = \mathop {\lim }\limits_{x \to 0} \dfrac{{{{\left( {3x + 1} \right)}^3} - {{\left( {1 - 4x} \right)}^4}}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{{{\left( {3x + 1} \right)}^3} - 1}}{x} - \mathop {\lim }\limits_{x \to 0} \dfrac{{{{\left( {1 - 4x} \right)}^4} - 1}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{3x\left[ {{{\left( {3x + 1} \right)}^2} + \left( {3x + 1} \right) + 1} \right]}}{x} - \mathop {\lim }\limits_{x \to 0} \dfrac{{ - 4x\left( {2 - 4x} \right)\left[ {{{\left( {1 - 4x} \right)}^2} + 1} \right]}}{x}\\ = \mathop {\lim }\limits_{x \to 0} 3\left[ {{{\left( {3x + 1} \right)}^2} + \left( {3x + 1} \right) + 1} \right] + \mathop {\lim }\limits_{x \to 0} 4\left( {2 - 4x} \right)\left[ {{{\left( {1 - 4x} \right)}^2} + 1} \right] = 25 \)

\( D = \mathop {\lim }\limits_{x \to 0} \dfrac{{\left( {1 + x} \right)\left( {1 + 2x} \right)\left( {1 + 3x} \right) - 1}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{\left( {1 + 2x + x + 2{x^2}} \right)\left( {1 + 3x} \right) - 1}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{{{\left( {1 + 3x + 2x} \right)}^2}\left( {1 + 3x} \right) - 1}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{6x + 11{x^2} + 6{x^3}}}{x}\\ = \mathop {\lim }\limits_{x \to 0} \dfrac{{x\left( {6 + 11x + 6{x^2}} \right)}}{x}\\ = \mathop {\lim }\limits_{x \to 0} 6 + 11x + 6{x^2} = 6 \)

\(a=\lim\limits_{x\rightarrow0}\frac{3x\left(1+x\right)\left(1+2x\right)}{x}+\lim\limits_{x\rightarrow0}\frac{2x\left(1+x\right)}{x}+\lim\limits_{x\rightarrow0}\frac{\left(1+x\right)-1}{x}\)

\(=\lim\limits_{x\rightarrow0}3\left(1+x\right)\left(1+2x\right)+\lim\limits_{x\rightarrow0}2\left(1+x\right)+1=3+2+1=6\)

\(b=\lim\limits_{x\rightarrow0}\frac{\left(x^5+5x^4+10x^3+10x^2+5x+1\right)-\left(1+5x\right)}{x^5+x^2}\)

\(=\lim\limits_{x\rightarrow0}\frac{x^2\left(x^3+5x^2+10\right)}{x^2\left(x^3+1\right)}=\lim\limits_{x\rightarrow0}\frac{x^3+5x^2+10}{x^3+1}=10\)

Đáp án A

Đó là nguyên lý của giới hạn kẹp

\(\left|f\left(x\right)\right|\le\left|x\right|\Rightarrow\lim\limits_{x\rightarrow0}f\left(x\right)=\lim\limits_{x\rightarrow0}x=0\)

Bài 1:

\(a=\lim\limits_{x\rightarrow-1}\frac{\left(x+1\right)\left(x^4-x^3+x^2-x+1\right)}{\left(x+1\right)\left(x^2-x+1\right)}=\lim\limits_{x\rightarrow-1}\frac{x^4-x^3+x^2-x+1}{x^2-x+1}=\frac{5}{3}\)

\(b=\frac{1-5+1}{0}=\frac{-3}{0}=-\infty\)

\(c=\lim\limits_{x\rightarrow1}\frac{x\left(1+2x\right)\left(1+3x\right)+2x\left(1+3x\right)+3x}{x}=\lim\limits_{x\rightarrow1}\left[\left(1+2x\right)\left(1+3x\right)+2\left(1+3x\right)+3\right]=1+2+3=6\)

\(d=\lim\limits_{x\rightarrow0}\frac{5\left(1+x\right)^4-1}{5x^4+2x}=\frac{4}{0}=+\infty\)

Bài 2:

\(a=\lim\limits_{x\rightarrow1}\frac{x^m-1}{x^n-1}=\lim\limits_{x\rightarrow1}\frac{mx^{m-1}}{nx^{n-1}}=\frac{m}{n}\)

\(b=\lim\limits_{x\rightarrow a}\frac{x-a}{x^n-a^n}=\lim\limits_{x\rightarrow a}\frac{1}{nx^{n-1}}=\frac{1}{n.a^{n-1}}\)

\(c=\lim\limits_{x\rightarrow0}\frac{x+x^2+...+x^n-n}{x-1}=\frac{-n}{-1}=n\)

\(\left(1+x\right)\left(1+2x\right)...\left(1+nx\right)=x\left(1+2x\right)...\left(1+nx\right)+\left(1+2x\right)\left(1+3x\right)...\left(1+nx\right)\)

\(=x\left(1+2x\right)...\left(1+nx\right)+2x\left(1+3x\right)...\left(1+nx\right)+\left(1+3x\right)...\left(1+nx\right)\)

\(=...\)

\(=x\left(1+2x\right)...\left(1+nx\right)+2x\left(1+3x\right)...\left(1+nx\right)+...+nx+1\)

\(\Rightarrow\lim\limits_{x\rightarrow0}\frac{\left(1+2x\right)\left(1+3x\right)...\left(1+nx\right)-1}{x}\)

\(=\lim\limits_{x\rightarrow0}\frac{x\left(1+2x\right)...\left(1+nx\right)+2x\left(1+3x\right)...\left(1+nx\right)+...+nx}{x}\)

\(=\lim\limits_{x\rightarrow0}\left[\left(1+2x\right)...\left(1+nx\right)+2\left(1+3x\right)...\left(1+nx\right)+...+n\right]\)

\(=1+2+3+...+n=\frac{n\left(n+1\right)}{2}\)

\(\lim\limits_{x\rightarrow0}\left(\dfrac{1}{x}-\dfrac{1}{x^2}\right)\)

\(=\lim\limits_{x\rightarrow0}\dfrac{x-1}{x^2}\)

\(=-\infty\) vì \(\left\{{}\begin{matrix}\lim\limits_{x\rightarrow0}x-1=0-1=-1< 0\\\lim\limits_{x\rightarrow0}x^2=0^2=0\end{matrix}\right.\)