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• (가)$\displaystyle \sum _ {n=3} ^ {\infty} \ln \left (1+ \frac{1} {n^2 -1} \right)$
• (나)$\displaystyle \sum _ {n=1} ^ {\infty} \frac{1} {n \left (1+ \frac{1} {\sqrt {2}} + \cdots + \frac{1} {\sqrt {n}} \right)}$
• (다)$\displaystyle \sum _ {n=1} ^ {\infty} \frac{1} {\sqrt {n}} \ln \left (\frac{n+1} {n} \right)$
• (라)$\displaystyle \sum _ {n=1} ^ {\infty} \sin \left (\frac{1} {n} \right)$