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@ -190,7 +190,7 @@ $$\begin{aligned}\lim_{x \to 3a} \frac{f(x)}{x-3a} &= \lim_{x \to 3a} \frac{\fra
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**答案**:$\displaystyle \lim_{x \to 3a} \frac{f(x)}{x-3a} = -\frac{1}{2}$.
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若级数$\sum\limits_{n=1}^{\infty}\frac{n^p}{(-1)^n}\sin(\frac{1}{\sqrt{n}})$绝对收敛,则常数$p$的取值范围是$\underline{\quad\quad\quad}.$
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首先,考虑级数$\sum\limits_{n=1}^{\infty}|\frac{n^p}{(-1)^n}\sin(\frac{1}{\sqrt{n}})|=\sum\limits_{n=1}^{\infty}\frac{n^p}{(-1)^n}|\sin(\frac{1}{\sqrt{n}})|$
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首先,考虑级数$\sum\limits_{n=1}^{\infty}|\frac{n^p}{(-1)^n}\sin(\frac{1}{\sqrt{n}})|=\sum\limits_{n=1}^{\infty}{n^p}|\sin(\frac{1}{\sqrt{n}})|$
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当$n\to\infty$时,$\frac{1}{\sqrt{n}}\to 0$,此时有等价无穷小关系:$\sin(\frac{1}{\sqrt{n}}) \sim \frac{1}{\sqrt{n}}.$
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因此,级数的通项可以近似为$\frac{1}{n^p}\frac{1}{\sqrt{n}}=\frac{1}{n^{p+\frac{1}{2}}}$
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根据**p级数**的收敛性结论,级数$\sum\limits_{n=1}^{\infty}\frac{1}{n^{p+\frac{1}{2}}}$当且仅当$p+\frac{1}{2}>1$时收敛,即$p>\frac{1}{2}$,
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