高等数学(第七版)同济大学 习题3 |
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高等数学(第七版)同济大学 习题3-2
1. 用洛必达法则求下列极限: \begin{aligned}&1. \ 用洛必达法则求下列极限:&\end{aligned} 1. 用洛必达法则求下列极限: ( 1 ) lim x → 0 l n ( 1 + x ) x ; ( 2 ) lim x → 0 e x − e − x s i n x ; ( 3 ) lim x → 0 t a n x − x x − s i n x ; ( 4 ) lim x → π s i n 3 x t a n 5 x ; ( 5 ) lim x → π 2 l n s i n x ( π − 2 x ) 2 ; ( 6 ) lim x → a x m − a m x n − a n ( a ≠ 0 ) ; ( 7 ) lim x → 0 + l n t a n 7 x l n t a n 2 x ; ( 8 ) lim x → π 2 t a n x t a n 3 x ; ( 9 ) lim x → + ∞ l n ( 1 + 1 x ) a r c c o t x ; ( 10 ) lim x → 0 l n ( 1 + x 2 ) s e c x − c o s x ; ( 11 ) lim x → 0 x c o t 2 x ; ( 12 ) lim x → 0 x 2 e 1 x 2 ; ( 13 ) lim x → 1 ( 2 x 2 − 1 − 1 x − 1 ) ; ( 14 ) lim x → ∞ ( 1 + a x ) x ; ( 15 ) lim x → 0 + x s i n x ; ( 16 ) lim x → 0 + ( 1 x ) t a n x \begin{aligned} &\ \ (1)\ \ \lim_{x \rightarrow 0}\frac{ln(1+x)}{x};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\ \ \lim_{x \rightarrow 0}\frac{e^x-e^{-x}}{sin\ x};\\\\ &\ \ (3)\ \ \lim_{x \rightarrow 0}\frac{tan\ x-x}{x-sin\ x};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)\ \ \lim_{x \rightarrow \pi}\frac{sin\ 3x}{tan\ 5x};\\\\ &\ \ (5)\ \ \lim_{x \rightarrow \frac{\pi}{2}}\frac{ln\ sin\ x}{(\pi-2x)^2};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (6)\ \ \lim_{x \rightarrow a}\frac{x^m-a^m}{x^n-a^n}\ (a \neq 0);\\\\ &\ \ (7)\ \ \lim_{x \rightarrow 0^+}\frac{ln\ tan\ 7x}{ln\ tan\ 2x};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (8)\ \ \lim_{x \rightarrow \frac{\pi}{2}}\frac{tan\ x}{tan\ 3x};\\\\ &\ \ (9)\ \ \lim_{x \rightarrow +\infty}\frac{ln\left(1+\frac{1}{x}\right)}{arccot\ x};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (10)\ \ \lim_{x \rightarrow 0}\frac{ln(1+x^2)}{sec\ x-cos\ x};\\\\ &\ \ (11)\ \ \lim_{x \rightarrow 0}xcot\ 2x;\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (12)\ \ \lim_{x \rightarrow 0}x^2e^{\frac{1}{x^2}};\\\\ &\ \ (13)\ \ \lim_{x \rightarrow 1}\left(\frac{2}{x^2-1}-\frac{1}{x-1}\right);\ \ \ \ \ \ \ \ \ \ \ \ \ \ (14)\ \ \lim_{x \rightarrow \infty}\left(1+\frac{a}{x}\right)^x;\\\\ &\ \ (15)\ \ \lim_{x \rightarrow 0^+}x^{sin\ x};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (16)\ \ \lim_{x \rightarrow 0^+}\left(\frac{1}{x}\right)^{tan\ x} & \end{aligned} (1) x→0limxln(1+x); (2) x→0limsin xex−e−x; (3) x→0limx−sin xtan x−x; (4) x→πlimtan 5xsin 3x; (5) x→2πlim(π−2x)2ln sin x; (6) x→alimxn−anxm−am (a=0); (7) x→0+limln tan 2xln tan 7x; (8) x→2πlimtan 3xtan x; (9) x→+∞limarccot xln(1+x1); (10) x→0limsec x−cos xln(1+x2); (11) x→0limxcot 2x; (12) x→0limx2ex21; (13) x→1lim(x2−12−x−11); (14) x→∞lim(1+xa)x; (15) x→0+limxsin x; (16) x→0+lim(x1)tan x 解:( 1 ) lim x → 0 l n ( 1 + x ) x = lim x → 0 1 1 + x = 1 ( 2 ) lim x → 0 e x − e − x s i n x = lim x → 0 e x + e − x c o s x = 2 ( 3 ) lim x → 0 t a n x − x x − s i n x = lim x → 0 s e c 2 x − 1 1 − c o s x = lim x → 0 1 + c o s x c o s 2 x = 2 ( 4 ) lim x → π s i n 3 x t a n 5 x = lim x → π 3 c o s 3 x 5 s e c 2 5 x = lim x → π 3 5 c o s 3 x ⋅ c o s 2 5 x = − 3 5 ( 5 ) lim x → π 2 l n s i n x ( π − 2 x ) 2 = lim x → π 2 c o s x s i n x 8 x − 4 π = lim x → π 2 c o t x 8 x − 4 π = lim x → π 2 − c s c 2 x 8 = − 1 8 ( 6 ) lim x → a x m − a m x n − a n = lim x → a m x m − 1 n x n − 1 = lim x → a m n x m − n = m n a m − n ( 7 ) lim x → 0 + l n t a n 7 x l n t a n 2 x = lim x → 0 + 7 s e c 2 7 x t a n 7 x 2 s e c 2 2 x t a n 2 x = lim x → 0 + 7 s i n 4 x 2 s i n 14 x = lim x → 0 + 28 c o s 4 x 28 c o s 14 x = 1 ( 8 ) lim x → π 2 t a n x t a n 3 x = lim x → π 2 s e c 2 x 3 s e c 2 3 x = lim x → π 2 c o s 2 3 x 3 c o s 2 x = lim x → π 2 − 6 c o s 3 x s i n 3 x − 6 c o s x s i n x = lim x → π 2 s i n 6 x s i n 2 x = lim x → π 2 6 c o s 6 x 2 c o s 2 x = 3 ( 9 ) lim x → + ∞ l n ( 1 + 1 x ) a r c c o t x = lim x → + ∞ − 1 x + x 2 − 1 1 + x 2 = lim x → + ∞ 1 + 1 x 2 1 + 1 x = 1 ( 10 ) lim x → 0 l n ( 1 + x 2 ) s e c x − c o s x = lim x → 0 l n ( 1 + x 2 ) s i n x t a n x = lim x → 0 2 x 1 + x 2 c o s x t a n x + s i n x s e c 2 x = lim x → 0 2 x 1 + x 2 s i n x + t a n x s e c x = lim x → 0 2 ( 1 − x 2 ) ( 1 + x 2 ) 2 c o s x + s e c 3 x + s e c x t a n 2 x = 1 ( 11 ) lim x → 0 x c o t 2 x = lim x → 0 x c o s 2 x s i n 2 x = lim x → 0 c o s 2 x − 2 x s i n 2 x 2 c o s 2 x = lim x → 0 ( 1 2 − x t a n 2 x ) = 1 2 ( 12 ) lim x → 0 x 2 e 1 x 2 = lim x → 0 e 1 x 2 1 x 2 = lim x → 0 e 1 x 2 = + ∞ ( 13 ) lim x → 1 ( 2 x 2 − 1 − 1 x − 1 ) = lim x → 1 2 − ( x + 1 ) ( x + 1 ) ( x − 1 ) = lim x → 1 − 1 x + 1 = − 1 2 ( 14 ) 令 1 t = a x ,则 x = t a ,当 x → ∞ 时, t → ∞ , lim x → ∞ ( 1 + a x ) x = lim t → ∞ ( 1 + 1 t ) t a = [ lim t → ∞ ( 1 + 1 t ) t ] a = e a ( 15 ) lim x → 0 + x s i n x = e lim x → 0 + s i n x l n x ,而 lim x → 0 + s i n x l n x = lim x → 0 + s i n x x ⋅ l n x 1 x = lim x → 0 + 1 x − 1 x 2 = lim x → 0 + ( − x ) = 0 , 所以 lim x → 0 + x s i n x = e 0 = 1 ( 16 ) lim x → 0 + ( 1 x ) t a n x = e lim x → 0 + t a n x l n 1 x ,而 lim x → 0 + t a n x l n 1 x = lim x → 0 + t a n x x ⋅ − l n x 1 x = lim x → 0 + − 1 x − 1 x 2 = 0 , 所以 lim x → 0 + ( 1 x ) t a n x = e 0 = 1 \begin{aligned} &\ \ (1)\ \lim_{x \rightarrow 0}\frac{ln(1+x)}{x}=\lim_{x \rightarrow 0}\frac{1}{1+x}=1\\\\ &\ \ (2)\ \lim_{x \rightarrow 0}\frac{e^x-e^{-x}}{sin\ x}=\lim_{x \rightarrow 0}\frac{e^x+e^{-x}}{cos\ x}=2\\\\ &\ \ (3)\ \lim_{x \rightarrow 0}\frac{tan\ x-x}{x-sin\ x}=\lim_{x \rightarrow 0}\frac{sec^2\ x-1}{1-cos\ x}=\lim_{x \rightarrow 0}\frac{1+cos\ x}{cos^2\ x}=2\\\\ &\ \ (4)\ \lim_{x \rightarrow \pi}\frac{sin\ 3x}{tan\ 5x}=\lim_{x \rightarrow \pi}\frac{3cos\ 3x}{5sec^2\ 5x}=\lim_{x \rightarrow \pi}\frac{3}{5}cos\ 3x \cdot cos^2\ 5x=-\frac{3}{5}\\\\ &\ \ (5)\ \lim_{x \rightarrow \frac{\pi}{2}}\frac{ln\ sin\ x}{(\pi-2x)^2}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{\frac{cos\ x}{sin\ x}}{8x-4\pi}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{cot\ x}{8x-4\pi}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{-csc^2\ x}{8}=-\frac{1}{8}\\\\ &\ \ (6)\ \lim_{x \rightarrow a}\frac{x^m-a^m}{x^n-a^n}=\lim_{x \rightarrow a}\frac{mx^{m-1}}{nx^{n-1}}=\lim_{x \rightarrow a}\frac{m}{n}x^{m-n}=\frac{m}{n}a^{m-n}\\\\ &\ \ (7)\ \lim_{x \rightarrow 0^+}\frac{ln\ tan\ 7x}{ln\ tan\ 2x}=\lim_{x \rightarrow 0^+}\frac{\frac{7sec^2\ 7x}{tan\ 7x}}{\frac{2sec^2\ 2x}{tan\ 2x}}=\lim_{x \rightarrow 0^+}\frac{7sin\ 4x}{2sin\ 14x}=\lim_{x \rightarrow 0^+}\frac{28cos\ 4x}{28cos\ 14x}=1\\\\ &\ \ (8)\ \lim_{x \rightarrow \frac{\pi}{2}}\frac{tan\ x}{tan\ 3x}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{sec^2\ x}{3sec^2\ 3x}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{cos^2\ 3x}{3cos^2\ x}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{-6cos\ 3xsin\ 3x}{-6cos\ xsin\ x}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{sin\ 6x}{sin\ 2x}=\lim_{x \rightarrow \frac{\pi}{2}}\frac{6cos\ 6x}{2cos\ 2x}=3\\\\ &\ \ (9)\ \lim_{x \rightarrow +\infty}\frac{ln\left(1+\frac{1}{x}\right)}{arccot\ x}=\lim_{x \rightarrow +\infty}\frac{-\frac{1}{x+x^2}}{-\frac{1}{1+x^2}}=\lim_{x \rightarrow +\infty}\frac{1+\frac{1}{x^2}}{1+\frac{1}{x}}=1\\\\ &\ \ (10)\ \lim_{x \rightarrow 0}\frac{ln(1+x^2)}{sec\ x-cos\ x}=\lim_{x \rightarrow 0}\frac{ln(1+x^2)}{sin\ x tan\ x}=\lim_{x \rightarrow 0}\frac{\frac{2x}{1+x^2}}{cos\ x tan\ x+sin\ x sec^2\ x}=\lim_{x \rightarrow 0}\frac{\frac{2x}{1+x^2}}{sin\ x+tan\ x sec\ x}=\\\\ &\ \ \ \ \ \ \ \ \ \ \lim_{x \rightarrow 0}\frac{\frac{2(1-x^2)}{(1+x^2)^2}}{cos\ x+sec^3\ x+sec\ xtan^2\ x}=1\\\\ &\ \ (11)\ \lim_{x \rightarrow 0}xcot\ 2x=\lim_{x \rightarrow 0}\frac{xcos\ 2x}{sin\ 2x}=\lim_{x \rightarrow 0}\frac{cos\ 2x-2xsin\ 2x}{2cos\ 2x}=\lim_{x \rightarrow 0}\left(\frac{1}{2}-xtan\ 2x\right)=\frac{1}{2}\\\\ &\ \ (12)\ \lim_{x \rightarrow 0}x^2e^{\frac{1}{x^2}}=\lim_{x \rightarrow 0}\frac{e^{\frac{1}{x^2}}}{\frac{1}{x^2}}=\lim_{x \rightarrow 0}e^{\frac{1}{x^2}}=+\infty\\\\ &\ \ (13)\ \lim_{x \rightarrow 1}\left(\frac{2}{x^2-1}-\frac{1}{x-1}\right)=\lim_{x \rightarrow 1}\frac{2-(x+1)}{(x+1)(x-1)}=\lim_{x \rightarrow 1}\frac{-1}{x+1}=-\frac{1}{2}\\\\ &\ \ (14)\ 令\frac{1}{t}=\frac{a}{x},则x=ta,当x \rightarrow \infty时,t \rightarrow \infty,\\\\ &\ \ \ \ \ \ \ \ \ \ \lim_{x \rightarrow \infty}\left(1+\frac{a}{x}\right)^x=\lim_{t \rightarrow \infty}\left(1+\frac{1}{t}\right)^{ta}=\left[\lim_{t \rightarrow \infty}\left(1+\frac{1}{t}\right)^t\right]^a=e^a\\\\ &\ \ (15)\ \lim_{x \rightarrow 0^+}x^{sin\ x}=e^{{\displaystyle \lim_{x \rightarrow 0^+}sin\ xln\ x}},而\lim_{x \rightarrow 0^+}sin\ xln\ x=\lim_{x \rightarrow 0^+}\frac{sin\ x}{x} \cdot \frac{ln\ x}{\frac{1}{x}}=\lim_{x \rightarrow 0^+}\frac{\frac{1}{x}}{-\frac{1}{x^2}}=\lim_{x \rightarrow 0^+}(-x)=0,\\\\ &\ \ \ \ \ \ \ \ \ \ \ 所以\lim_{x \rightarrow 0^+}x^{sin\ x}=e^0=1\\\\ &\ \ (16)\ \lim_{x \rightarrow 0^+}\left(\frac{1}{x}\right)^{tan\ x}=e^{{\displaystyle \lim_{x \rightarrow 0^+}tan\ xln\ \frac{1}{x}}},而\lim_{x \rightarrow 0^+}tan\ xln\ \frac{1}{x}=\lim_{x \rightarrow 0^+}\frac{tan\ x}{x}\cdot \frac{-ln\ x}{\frac{1}{x}}=\lim_{x \rightarrow 0^+}\frac{-\frac{1}{x}}{-\frac{1}{x^2}}=0,\\\\ &\ \ \ \ \ \ \ \ \ \ \ 所以\lim_{x \rightarrow 0^+}\left(\frac{1}{x}\right)^{tan\ x}=e^0=1 & \end{aligned} (1) x→0limxln(1+x)=x→0lim1+x1=1 (2) x→0limsin xex−e−x=x→0limcos xex+e−x=2 (3) x→0limx−sin xtan x−x=x→0lim1−cos xsec2 x−1=x→0limcos2 x1+cos x=2 (4) x→πlimtan 5xsin 3x=x→πlim5sec2 5x3cos 3x=x→πlim53cos 3x⋅cos2 5x=−53 (5) x→2πlim(π−2x)2ln sin x=x→2πlim8x−4πsin xcos x=x→2πlim8x−4πcot x=x→2πlim8−csc2 x=−81 (6) x→alimxn−anxm−am=x→alimnxn−1mxm−1=x→alimnmxm−n=nmam−n (7) x→0+limln tan 2xln tan 7x=x→0+limtan 2x2sec2 2xtan 7x7sec2 7x=x→0+lim2sin 14x7sin 4x=x→0+lim28cos 14x28cos 4x=1 (8) x→2πlimtan 3xtan x=x→2πlim3sec2 3xsec2 x=x→2πlim3cos2 xcos2 3x=x→2πlim−6cos xsin x−6cos 3xsin 3x=x→2πlimsin 2xsin 6x=x→2πlim2cos 2x6cos 6x=3 (9) x→+∞limarccot xln(1+x1)=x→+∞lim−1+x21−x+x21=x→+∞lim1+x11+x21=1 (10) x→0limsec x−cos xln(1+x2)=x→0limsin xtan xln(1+x2)=x→0limcos xtan x+sin xsec2 x1+x22x=x→0limsin x+tan xsec x1+x22x= x→0limcos x+sec3 x+sec xtan2 x(1+x2)22(1−x2)=1 (11) x→0limxcot 2x=x→0limsin 2xxcos 2x=x→0lim2cos 2xcos 2x−2xsin 2x=x→0lim(21−xtan 2x)=21 (12) x→0limx2ex21=x→0limx21ex21=x→0limex21=+∞ (13) x→1lim(x2−12−x−11)=x→1lim(x+1)(x−1)2−(x+1)=x→1limx+1−1=−21 (14) 令t1=xa,则x=ta,当x→∞时,t→∞, x→∞lim(1+xa)x=t→∞lim(1+t1)ta=[t→∞lim(1+t1)t]a=ea (15) x→0+limxsin x=ex→0+limsin xln x,而x→0+limsin xln x=x→0+limxsin x⋅x1ln x=x→0+lim−x21x1=x→0+lim(−x)=0, 所以x→0+limxsin x=e0=1 (16) x→0+lim(x1)tan x=ex→0+limtan xln x1,而x→0+limtan xln x1=x→0+limxtan x⋅x1−ln x=x→0+lim−x21−x1=0, 所以x→0+lim(x1)tan x=e0=1 2. 验证极限 lim x → ∞ x + s i n x x 存在,但不能用洛必达法则得出。 \begin{aligned}&2. \ 验证极限\lim_{x \rightarrow \infty}\frac{x+sin\ x}{x}存在,但不能用洛必达法则得出。&\end{aligned} 2. 验证极限x→∞limxx+sin x存在,但不能用洛必达法则得出。 解:lim x → ∞ x + s i n x x = lim x → ∞ ( 1 + s i n x x ) = 1 \begin{aligned} &\ \ \lim_{x \rightarrow \infty}\frac{x+sin\ x}{x}=\lim_{x \rightarrow \infty}\left(1+\frac{sin\ x}{x}\right)=1 & \end{aligned} x→∞limxx+sin x=x→∞lim(1+xsin x)=1 3. 验证极限 lim x → 0 x 2 s i n 1 x s i n x 存在,但不能用洛必达法则得出。 \begin{aligned}&3. \ 验证极限\lim_{x \rightarrow 0}\frac{x^2sin\ \frac{1}{x}}{sin\ x}存在,但不能用洛必达法则得出。&\end{aligned} 3. 验证极限x→0limsin xx2sin x1存在,但不能用洛必达法则得出。 解:lim x → 0 x 2 s i n 1 x s i n x = lim x → 0 x s i n 1 x s i n x x = 0 \begin{aligned} &\ \ \lim_{x \rightarrow 0}\frac{x^2sin\ \frac{1}{x}}{sin\ x}=\lim_{x \rightarrow 0}\frac{xsin\ \frac{1}{x}}{\frac{sin\ x}{x}}=0 & \end{aligned} x→0limsin xx2sin x1=x→0limxsin xxsin x1=0 4. 讨论函数 f ( x ) = { [ ( 1 + x ) 1 x e ] 1 x , x > 0 , e − 1 2 , x ≤ 0 在点 x = 0 处的连续性。 \begin{aligned}&4. \ 讨论函数f(x)=\begin{cases}\left[\frac{(1+x)^{\frac{1}{x}}}{e}\right]^{\frac{1}{x}},x \gt 0,\\\\e^{-\frac{1}{2}},\ \ \ \ \ \ \ \ \ \ \ \ x \le 0\end{cases}在点x=0处的连续性。&\end{aligned} 4. 讨论函数f(x)=⎩ ⎨ ⎧[e(1+x)x1]x1,x>0,e−21, x≤0在点x=0处的连续性。 解:lim x → 0 + f ( x ) = lim x → 0 + [ ( 1 + x ) 1 x e ] 1 x = e lim x → 0 + 1 x l n [ ( 1 + x ) 1 x e ] , 而 lim x → 0 + 1 x l n [ 1 x l n ( 1 + x ) − 1 ] = lim x → 0 + l n ( 1 + x ) − x x 2 = lim x → 0 + 1 1 + x − 1 2 x = lim x → 0 + − 1 2 ( 1 + x ) = − 1 2 ,所以 lim x → 0 + f ( x ) = e − 1 2 , 因 lim x → 0 − f ( x ) = lim x → 0 − e − 1 2 = e − 1 2 , f ( 0 ) = e − 1 2 ,所以 lim x → 0 + f ( x ) = lim x → 0 − f ( x ) = f ( 0 ) ,函数 f ( x ) 在 x = 0 处连续。 \begin{aligned} &\ \ \lim_{x \rightarrow 0^+}f(x)=\lim_{x \rightarrow 0^+}\left[\frac{(1+x)^{\frac{1}{x}}}{e}\right]^{\frac{1}{x}}=e^{{\displaystyle \lim_{x \rightarrow 0^+}\frac{1}{x}ln\left[\frac{(1+x)^{\frac{1}{x}}}{e}\right]}},\\\\ &\ \ 而\lim_{x \rightarrow 0^+}\frac{1}{x}ln\left[\frac{1}{x}ln(1+x)-1\right]=\lim_{x \rightarrow 0^+}\frac{ln(1+x)-x}{x^2}=\lim_{x \rightarrow 0^+}\frac{\frac{1}{1+x}-1}{2x}=\lim_{x \rightarrow 0^+}-\frac{1}{2(1+x)}=-\frac{1}{2},所以\lim_{x \rightarrow 0^+}f(x)=e^{-\frac{1}{2}},\\\\ &\ \ 因\lim_{x \rightarrow 0^-}f(x)=\lim_{x \rightarrow 0^-}e^{-\frac{1}{2}}=e^{-\frac{1}{2}},f(0)=e^{-\frac{1}{2}},所以\lim_{x \rightarrow 0^+}f(x)=\lim_{x \rightarrow 0^-}f(x)=f(0),函数f(x)在x=0处连续。 & \end{aligned} x→0+limf(x)=x→0+lim[e(1+x)x1]x1=ex→0+limx1ln[e(1+x)x1], 而x→0+limx1ln[x1ln(1+x)−1]=x→0+limx2ln(1+x)−x=x→0+lim2x1+x1−1=x→0+lim−2(1+x)1=−21,所以x→0+limf(x)=e−21, 因x→0−limf(x)=x→0−lime−21=e−21,f(0)=e−21,所以x→0+limf(x)=x→0−limf(x)=f(0),函数f(x)在x=0处连续。 |
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