Cách làm cho bạn:
$(\frac{1}{2}-1) (\frac{1}{3}-1)(\frac{1}{4}-1)...(\frac{1}{1000}-1)$
$= (\frac{1}{2}-\frac{2}{2}) (\frac{1}{3}-\frac{3}{3})(\frac{1}{4}-\frac{4}{4})...(\frac{1}{1000}-\frac{1000}{1000})$
$= \frac{-1}{2}\times \frac{-2}{3}\times \frac{-3}{4}\times ...\times \frac{-999}{1000}$
$= \frac{-1}{1000}$
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