已知级数:①\(\sum_{n = 1}^{\infty}\sin\frac{n^{3}\pi}{n^{2}+1}\);②\(\sum_{n = 1}^{\infty}(-1)^{n}\left(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\right)\),则A. ①与②均条件收敛. B. ①条件收敛,②绝对收敛. C. ①绝对收敛,②条件收敛. D. ①与②均绝对收敛.
【答案】B
【解...
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【解析】 \(\left|\sin\frac{n^{3}\pi}{n^{2}+1}\right|=\left|\sin\left(\frac{n^{3}\pi}{n^{2}+1}-n\pi\right)\right|=\left|\sin\frac{n}{n^{2}+1}\pi\right|\sim\frac{n}{n^{2}+1}\pi\sim\frac{1}{n}\pi\) . \(\sum_{n = 1}^{\infty}\frac{1}{n}\)发散 \(\therefore\)不是绝对收敛.
\(\sin\frac{n^{3}\pi}{n^{2}+1}=(-1)^{n}\sin\left(\frac{n^{3}\pi}{n^{2}+1}-n\pi\right)=(-1)^{n}\sin\frac{n}{n^{2}+1}\pi\),为交错级数.
\(\sin\frac{n}{n^{2}+1}\pi\)递减,\(\therefore\)条件收敛.
\(\sum_{n = 1}^{\infty}(-1)^{n}\left(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\right)\) .
\(\left|(-1)^{n}\left(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\right)\right|=\left|-\frac{1}{3}\frac{1}{n^{\frac{2}{3}\times\frac{3}{2}}}+o\left(\frac{1}{n^{2}}\right)\right|\)(注:原解析此处\(n\)的次数书写可能有误,推测应为\(\left|-\frac{1}{3}\frac{1}{n^{\frac{4}{3}}}+o\left(\frac{1}{n^{\frac{4}{3}}}\right)\right|\) ,按你提供内容提取 ),实际准确是利用等价无穷小,当\(x\to0\)时,\(x - \tan x\sim-\frac{1}{3}x^{3}\),令\(x = \frac{1}{\sqrt[3]{n^{2}}}=n^{-\frac{2}{3}}\),则\(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\sim-\frac{1}{3}n^{-2}\),所以\(\left|(-1)^{n}\left(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\right)\right|\sim\frac{1}{3n^{2}}\) )
\(\sum_{n = 1}^{\infty}\frac{1}{n^{2}}\)收敛 \(\therefore\sum_{n = 1}^{\infty}(-1)^{n}\left(\frac{1}{\sqrt[3]{n^{2}}}-\tan\frac{1}{\sqrt[3]{n^{2}}}\right)\) 绝对收敛
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