[ง่วงนอน]

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4.\sum_{n = 1}^{\infty} \frac{n+1}{n!}=\sum_{n = 1}^{\infty}( \frac{1}{(n-1)!}+ \frac{1}{n!} )=1+\frac{1}{1!} +\frac{1}{2!} +\frac{1}{3!} +......=1+e


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