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#1
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àÃ×èͧ͹ءÃÁ §§ ÁÒ¡¤ÃѺ!!!
\sum_{n = 1}^{\infty} 1/(n(n+4))
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#2
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\begin{align*}
\dfrac{1}{n(n+4)} &=\dfrac{1}{4}\left(\dfrac{1}{n}-\dfrac{1}{n+4}\right)\\ &=\dfrac{1}{4}\left(\dfrac{1}{n}+\dfrac{1}{n+1}+\dfrac{1}{n+2}+\dfrac{1}{n+3}\right)-\dfrac{1}{4}\left(\dfrac{1}{n+1}+\dfrac{1}{n+2}+\dfrac{1}{n+3}+\dfrac{1}{n+4}\right) \end{align*} µÍº $\dfrac{25}{12}$
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