S
snowangel1103


Tính
$\frac{1}{3}+\frac{1}{3+6}+\frac{1}{3+6+9}+...+ \frac{1}{3+6+9+...+2013} $
$\frac{1}{3}+\frac{1}{3+6}+\frac{1}{3+6+9}+...+ \frac{1}{3+6+9+...+2013} $
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