解
lim x → 0 sin x + x 2 sin 1 x ( 2 + x 2 ) ln ( 1 + x ) = lim x → 0 1 2 + x 2 ⋅ sin x + x 2 sin 1 x x = 1 2 lim x → 0 ( sin x x + x sin 1 x ) = 1 2 . \begin{aligned} \lim\limits_{x\to0}\cfrac{\sin x+x^2\sin\cfrac{1}{x}}{(2+x^2)\ln(1+x)}&=\lim\limits_{x\to0}\cfrac{1}{2+x^2}\cdot\cfrac{\sin x+x^2\sin\cfrac{1}{x}}{x}\\ &=\cfrac{1}{2}\lim\limits_{x\to0}\left(\cfrac{\sin x}{x}+x\sin\cfrac{1}{x}\right)=\cfrac{1}{2}. \end{aligned} x→0lim(2+x2)ln(1+x)sinx+x2sinx1=x→0lim2+x21⋅xsinx+x2sinx1=21x→0lim(xsinx+xsinx1)=21.
(这道题主要利用了洛必达法则适用条件求解)
解 选项 ( A ) (A) (A), 1 + x − 1 ∼ 1 2 x \sqrt{1+x}-1\sim\cfrac{1}{2}x 1+x−1∼21x,是关于 x x x的一阶无穷小。
选项 ( B ) (B) (B), ln ( 1 + x ) − x = [ x − 1 2 x 2 + ο ( x 2 ) ] − x ∼ − 1 2 x 2 \ln(1+x)-x=\left[x-\cfrac{1}{2}x^2+\omicron(x^2)\right]-x\sim-\cfrac{1}{2}x^2 ln(1+x)−x=[x−21x2+ο(x2)]−x∼−21x2,是关于 x x x的二阶无穷小。
选项 ( C ) (C) (C), cos ( sin x ) − 1 ∼ − 1 2 sin 2 x ∼ − 1 2 x 2 \cos(\sin x)-1\sim-\cfrac{1}{2}\sin^2x\sim-\cfrac{1}{2}x^2 cos(sinx)−1∼−21sin2x∼−21x2,是关于 x x x的二阶无穷小。
选项 ( D ) (D) (D), x x − 1 = e x ln x − 1 ∼ x ln x x^x-1=e^{x\ln x}-1\sim x\ln x xx−1=exlnx−1∼xlnx,不是关于 x x x的一阶无穷小。(这道题主要利用了无穷小求解)
解
lim x → ∞ e − x ( 1 + 1 x ) x 2 = lim x → ∞ e x 2 ln ( 1 + 1 x ) − x = e lim x → ∞ ln ( 1 + 1 x ) − 1 x ( 1 x ) 2 = x = 1 t e lim t → 0 ln ( 1 + t ) − t t 2 = e lim t → 0 1 1 + t − 1 2 t = e − 1 2 . \begin{aligned} \lim\limits_{x\to\infty}e^{-x}\left(1+\cfrac{1}{x}\right)^{x^2}&=\lim\limits_{x\to\infty}e^{x^2\ln\left(1+\frac{1}{x}\right)-x}=e^{\lim\limits_{x\to\infty}\dfrac{\ln\left(1+\frac{1}{x}\right)-\frac{1}{x}}{\left(\frac{1}{x}\right)^2}}\\ &\xlongequal{x=\frac{1}{t}}e^{\lim\limits_{t\to0}\frac{\ln(1+t)-t}{t^2}}=e^{\lim\limits_{t\to0}\frac{\frac{1}{1+t}-1}{2t}}=e^{-\frac{1}{2}}. \end{aligned} x→∞lime−x(1+x1)x2=x→∞limex2ln(1+x1)−x=ex→∞lim(x1)2ln(1+x1)−x1x=t1et→0limt2ln(1+t)−t=et→0lim2t1+t1−1=e−21.
(这道题主要利用了变量代换求解)
解 因为 lim x → 0 + x x = lim x → 0 + e x ln x = 1 , lim x → 0 + e x ln tan x = 1 \lim\limits_{x\to0^+}x^x=\lim\limits_{x\to0^+}e^{x\ln x}=1,\lim\limits_{x\to0^+}e^{x\ln\tan x}=1 x→0+limxx=x→0+limexlnx=1,x→0+limexlntanx=1。对分母作等价无穷小代换: 1 + 3 sin 2 x − 1 ∼ 1 2 ⋅ 3 sin 2 x ∼ 3 2 x 2 ( x → 0 + ) \sqrt{1+3\sin^2x}-1\sim\cfrac{1}{2}\cdot3\sin^2x\sim\cfrac{3}{2}x^2(x\to0^+) 1+3sin2x−1∼21⋅3sin2x∼23x2(x→0+),得
原式 = 2 3 lim x → 0 + x x − ( tan x ) x x 3 = 2 3 lim x → 0 + x x [ 1 − ( tan x x ) x ] x 3 = − 2 3 lim x → 0 + ( tan x x ) x − 1 x 3 = − 2 3 lim x → 0 + e x ln ( tan x x ) − 1 x 3 . \begin{aligned} \text{原式}&=\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{x^x-(\tan x)^x}{x^3}=\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{x^x\left[1-\left(\cfrac{\tan x}{x}\right)^x\right]}{x^3}\\ &=-\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{\left(\cfrac{\tan x}{x}\right)^x-1}{x^3}=-\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{e^{x\ln\left(\frac{\tan x}{x}\right)}-1}{x^3}. \end{aligned} 原式=32x→0+limx3xx−(tanx)x=32x→0+limx3xx[1−(xtanx)x]=−32x→0+limx3(xtanx)x−1=−32x→0+limx3exln(xtanx)−1.
对分子作等价无穷小代换:当 x → 0 + x\to0^+ x→0+时,有
e x ln ( tan x x ) − 1 ∼ x ln ( tan x x ) = x ln [ 1 + ( tan x x − 1 ) ] ∼ x ( tan x x − 1 ) = tan x − x , 原式 = − 2 3 lim x → 0 + tan x − x x 3 = − 2 3 lim x → 0 + sec 2 x − 1 3 x 2 = − 4 9 lim x → 0 + ( tan x x ) 2 = − 2 9 . e^{x\ln\left(\frac{\tan x}{x}\right)}-1\sim x\ln\left(\cfrac{\tan x}{x}\right)=x\ln\left[1+\left(\frac{\tan x}{x}-1\right)\right]\sim x\left(\frac{\tan x}{x}-1\right)=\tan x-x,\\ \begin{aligned} \text{原式}&=-\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{\tan x-x}{x^3}=-\cfrac{2}{3}\lim\limits_{x\to0^+}\cfrac{\sec^2x-1}{3x^2}\\ &=-\cfrac{4}{9}\lim\limits_{x\to0^+}\left(\cfrac{\tan x}{x}\right)^2=-\cfrac{2}{9}. \end{aligned} exln(xtanx)−1∼xln(xtanx)=xln[1+(xtanx−1)]∼x(xtanx−1)=tanx−x,原式=−32x→0+limx3tanx−x=−32x→0+lim3x2sec2x−1=−94x→0+lim(xtanx)2=−92.
(这道题主要利用了等价无穷小代换求解)
解 首先,显然有 lim x → ∞ f ( x ) x 3 = lim x → ∞ ( 27 + 5 x − 2 x 3 ) = 27 \lim\limits_{x\to\infty}\cfrac{f(x)}{x^3}=\lim\limits_{x\to\infty}\left(27+\cfrac{5}{x}-\cfrac{2}{x^3}\right)=27 x→∞limx3f(x)=x→∞lim(27+x5−x32)=27。
先令 t = 27 x t=27x t=27x,再令 y = f − 1 ( x ) y=f^{-1}(x) y=f−1(x),则 t = f ( y ) t=f(y) t=f(y),且 x → ∞ x\to\infty x→∞时, t → ∞ , y → ∞ t\to\infty,y\to\infty t→∞,y→∞,所以
lim x → ∞ f − 1 ( 27 x ) x 3 = 3 lim t → ∞ f − 1 ( t ) t 3 = 3 lim y → ∞ y f ( y ) 3 = 3 lim y → ∞ y 3 f ( y ) 3 = 3 1 27 3 = 1. \begin{aligned} \lim\limits_{x\to\infty}\cfrac{f^{-1}(27x)}{\sqrt[3]{x}}&=3\lim\limits_{t\to\infty}\cfrac{f^{-1}(t)}{\sqrt[3]{t}}=3\lim\limits_{y\to\infty}\cfrac{y}{\sqrt[3]{f(y)}}=3\sqrt[3]{\lim\limits_{y\to\infty}\cfrac{y^3}{f(y)}}\\ &=3\sqrt[3]{\cfrac{1}{27}}=1. \end{aligned} x→∞lim3xf−1(27x)=3t→∞lim3tf−1(t)=3y→∞lim3f(y)y=33y→∞limf(y)y3=33271=1.
同理,令 y = f − 1 ( x ) y=f^{-1}(x) y=f−1(x),则 x = f ( y ) x=f(y) x=f(y),且 x → ∞ x\to\infty x→∞时, y → ∞ y\to\infty y→∞,所以
lim x → ∞ f − 1 ( x ) x 3 = lim y → ∞ y f ( y ) 3 = lim y → ∞ y 3 f ( y ) 3 = 1 27 3 = 1 3 . \lim\limits_{x\to\infty}\cfrac{f^{-1}(x)}{\sqrt[3]{x}}=\lim\limits_{y\to\infty}\cfrac{y}{\sqrt[3]{f(y)}}=\sqrt[3]{\lim\limits_{y\to\infty}\cfrac{y^3}{f(y)}}=\sqrt[3]{\cfrac{1}{27}}=\cfrac{1}{3}. x→∞lim3xf−1(x)=y→∞lim3f(y)y=3y→∞limf(y)y3=3271=31.
因此,原式 = 1 − 1 3 = 2 3 =1-\cfrac{1}{3}=\cfrac{2}{3} =1−31=32。(这道题主要利用了变量代换求解)
解
lim x → 0 ∫ 0 x sin 2 t 4 + t 2 ∫ 0 x ( t + 1 − 1 ) d t = lim x → 0 sin 2 x 4 + x 2 ( x + 1 − 1 ) = lim x → 0 ( x + 1 + 1 ) sin 2 x x 4 + x 2 = lim x → 0 2 ( x + 1 + 1 ) 4 + x 2 ⋅ sin 2 x 2 x = 2. \begin{aligned} \lim\limits_{x\to0}\displaystyle\int^x_0\cfrac{\sin2t}{\sqrt{4+t^2}\displaystyle\int^x_0(\sqrt{t+1}-1)\mathrm{d}t}&=\lim\limits_{x\to0}\cfrac{\sin2x}{\sqrt{4+x^2}(\sqrt{x+1}-1)}\\ &=\lim\limits_{x\to0}\cfrac{(\sqrt{x+1}+1)\sin2x}{x\sqrt{4+x^2}}\\ &=\lim\limits_{x\to0}\cfrac{2(\sqrt{x+1}+1)}{\sqrt{4+x^2}}\cdot\cfrac{\sin 2x}{2x}\\ &=2. \end{aligned} x→0lim∫0x4+t2∫0x(t+1−1)dtsin2t=x→0lim4+x2(x+1−1)sin2x=x→0limx4+x2(x+1+1)sin2x=x→0lim4+x22(x+1+1)⋅2xsin2x=2.
(这道题主要利用了积分式求导求解)
解 令 x = 1 t x=\cfrac{1}{t} x=t1,有
lim x → + ∞ ( x 3 + 2 x 2 + 1 3 − x e 1 x ) = lim t → 0 + t 3 + 2 t + 1 3 − e t t = lim t → 0 + 1 3 ( t 3 + 2 t + 1 ) − 2 3 ( 2 + 3 t 2 ) − e t 1 = 1 3 × 1 × 2 − 1 = − 1 3 . \begin{aligned} \lim\limits_{x\to+\infty}(\sqrt[3]{x^3+2x^2+1}-xe^{\frac{1}{x}})&=\lim\limits_{t\to0^+}\cfrac{\sqrt[3]{t^3+2t+1}-e^t}{t}\\ &=\lim\limits_{t\to0^+}\cfrac{\cfrac{1}{3}(t^3+2t+1)^{-\frac{2}{3}}(2+3t^2)-e^t}{1}\\ &=\cfrac{1}{3}\times1\times2-1=-\cfrac{1}{3}. \end{aligned} x→+∞lim(3x3+2x2+1−xex1)=t→0+limt3t3+2t+1−et=t→0+lim131(t3+2t+1)−32(2+3t2)−et=31×1×2−1=−31.
(这道题主要利用了变量代换求解)
解
lim x → 0 1 + 1 2 x 2 − 1 + x 2 ( cos x − e x 2 2 ) sin s 2 2 = lim x → 0 1 + 1 2 x 2 − [ 1 + 1 2 x 2 − 1 8 x 4 + ο ( x 4 ) ] { [ 1 − 1 2 x 2 + ο ( x 2 ) ] − [ 1 + 1 2 x 2 + ο ( x 2 ) ] } ⋅ x 2 2 = lim x → 0 1 8 x 4 + ο ( x 4 ) − 1 2 x 2 + ο ( x 4 ) = − 1 4 . \begin{aligned} \lim\limits_{x\to0}\cfrac{1+\cfrac{1}{2}x^2-\sqrt{1+x^2}}{(\cos x-e^{\frac{x^2}{2}})\sin\cfrac{s^2}{2}}&=\lim\limits_{x\to0}\cfrac{1+\cfrac{1}{2}x^2-\left[1+\cfrac{1}{2}x^2-\cfrac{1}{8}x^4+\omicron(x^4)\right]}{\left\{\left[1-\cfrac{1}{2}x^2+\omicron(x^2)\right]-\left[1+\cfrac{1}{2}x^2+\omicron(x^2)\right]\right\}\cdot\cfrac{x^2}{2}}\\ &=\lim\limits_{x\to0}\cfrac{\cfrac{1}{8}x^4+\omicron(x^4)}{-\cfrac{1}{2}x^2+\omicron(x^4)}=-\cfrac{1}{4}. \end{aligned} x→0lim(cosx−e2x2)sin2s21+21x2−1+x2=x→0lim{[1−21x2+ο(x2)]−[1+21x2+ο(x2)]}⋅2x21+21x2−[1+21x2−81x4+ο(x4)]=x→0lim−21x2+ο(x4)81x4+ο(x4)=−41.
(这道题主要利用了泰勒展开式求解)
解 当 k ⩽ 0 k\leqslant0 k⩽0时, I = − ∞ I=-\infty I=−∞,极限不存在;
当 k > 0 k>0 k>0时,
I = x = 1 t ∞ − ∞ lim t → 0 + [ ( 1 t α + 8 t 4 + 2 ) k − 1 t ] ( α ⩾ 5 ) = lim t → 0 + ( 1 + 8 t α − 4 + 2 t α ) k − t α k − 1 t α k . \begin{aligned} I&\xlongequal[x=\frac{1}{t}]{\infty-\infty}\lim\limits_{t\to0^+}\left[\left(\frac{1}{t^\alpha}+\frac{8}{t^4}+2\right)^k-\frac{1}{t}\right](\alpha\geqslant5)\\ &=\lim\limits_{t\to0^+}\cfrac{(1+8t^{\alpha-4}+2t^\alpha)^{k}-t^{\alpha k-1}}{t^{\alpha k}}. \end{aligned} I∞−∞x=t1t→0+lim[(tα1+t48+2)k−t1](α⩾5)=t→0+limtαk(1+8tα−4+2tα)k−tαk−1.
只有当 α k − 1 = 0 \alpha k-1=0 αk−1=0,即 k = 1 α k=\cfrac{1}{\alpha} k=α1时极限才可以用洛必达法则,否则极限为 ∞ \infty ∞,不存在。故
I = lim t → 0 + ( 1 + 8 t α − 4 + 2 t α ) 1 α − 1 t = lim t → 0 + 1 α ( 8 t α − 4 + 2 t α ) t . I=\lim\limits_{t\to0^+}\cfrac{(1+8t^{\alpha-4}+2t^\alpha)^{\frac{1}{\alpha}}-1}{t}=\lim\limits_{t\to0^+}\cfrac{\cfrac{1}{\alpha}(8t^{\alpha-4}+2t^\alpha)}{t}. I=t→0+limt(1+8tα−4+2tα)α1−1=t→0+limtα1(8tα−4+2tα).
当 α = 5 \alpha=5 α=5时, I = lim t → 0 + 1 5 8 t + 2 t 5 t = 8 5 I=\lim\limits_{t\to0^+}\cfrac{1}{5}\cfrac{8t+2t^5}{t}=\cfrac{8}{5} I=t→0+lim51t8t+2t5=58,此时 k = 1 5 k=\cfrac{1}{5} k=51;
当 α > 5 \alpha>5 α>5时, I = 0 I=0 I=0,此时 k = 1 5 k=\cfrac{1}{5} k=51。(这道题主要利用了分类讨论求解)
解 当 0 ⩽ x < 1 2 0\leqslant x<\cfrac{1}{2} 0⩽x<21时,有 1 n ⩽ 1 + ( 2 x ) n + x 2 n n ⩽ 1 ⋅ 3 n \sqrt[n]{1}\leqslant\sqrt[n]{1+(2x)^n+x^{2n}}\leqslant1\cdot\sqrt[n]{3} n1⩽n1+(2x)n+x2n⩽1⋅n3;
当 1 2 ⩽ x < 2 \cfrac{1}{2}\leqslant x<2 21⩽x<2时,有 2 x < 1 + ( 2 x ) n + x 2 n n ⩽ 2 x 3 n 2x<\sqrt[n]{1+(2x)^n+x^{2n}}\leqslant2x\sqrt[n]{3} 2x<n1+(2x)n+x2n⩽2xn3;
当 x ⩾ 2 x\geqslant2 x⩾2时,有 x 2 ⩽ 1 + ( 2 x ) n + x 2 n n ⩽ x 2 3 n x^2\leqslant\sqrt[n]{1+(2x)^n+x^{2n}}\leqslant x^2\sqrt[n]{3} x2⩽n1+(2x)n+x2n⩽x2n3。
又 lim n → ∞ 3 n = 1 \lim\limits_{n\to\infty}\sqrt[n]{3}=1 n→∞limn3=1,故
f ( x ) = { 1 , 0 ⩽ x < 1 2 , 2 x , 1 2 ⩽ x < 2 , x 2 , x ⩾ 2. f(x)=\begin{cases} 1,&0\leqslant x<\cfrac{1}{2},\\ 2x,&\cfrac{1}{2}\leqslant x<2,\\ x^2,&x\geqslant2. \end{cases} f(x)=⎩⎪⎪⎪⎪⎨⎪⎪⎪⎪⎧1,2x,x2,0⩽x<21,21⩽x<2,x⩾2.
解 因为 f ( x ) f(x) f(x)在 [ 0 , 1 2 ) , [ 1 2 , 2 ) , [ 2 , + ∞ ) \left[0,\cfrac{1}{2}\right),\left[\cfrac{1}{2},2\right),\left[2,+\infty\right) [0,21),[21,2),[2,+∞)上连续,又
lim x → ( 1 2 ) − f ( x ) = 1 , lim x → ( 1 2 ) + f ( x ) = 1 , f ( 1 2 ) = 1 , lim x → 2 − f ( x ) = 4 , lim x → 2 + f ( x ) = 4 , f ( 2 ) = 4 , \lim\limits_{x\to\left(\frac{1}{2}\right)^-}f(x)=1,\quad\lim\limits_{x\to\left(\frac{1}{2}\right)^+}f(x)=1,\quad f\left(\frac{1}{2}\right)=1,\\ \lim\limits_{x\to2^-}f(x)=4,\quad\lim\limits_{x\to2^+}f(x)=4,\quad f(2)=4,\\ x→(21)−limf(x)=1,x→(21)+limf(x)=1,f(21)=1,x→2−limf(x)=4,x→2+limf(x)=4,f(2)=4,
所以 f ( x ) f(x) f(x)在 [ 0 , + ∞ ) [0,+\infty) [0,+∞)上连续。(这道题主要利用了分类讨论求解)
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