Sum an = 1 / 3N (n + 1), Sn

an=(1/3)[1/n-1/(n+1)]
sn=(1/3){(1/1-1/2)+(1/2-1/3)+.+[1/n-1/(n+1)]}
=(1/3)[1-1/(n+1)]
=1/3-1/3(n+1)

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