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khaiproqn81

$(1-\dfrac{1}{2})(1-\dfrac{1}{3})(1-\dfrac{1}{4})...(1-\dfrac{1}{n+1}) \\ =\dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}...\dfrac{n-1}{n}.\dfrac{n}{n+1} \\ =\dfrac{1.2.3...(n-1).n}{2.3.4...n.(n+1)} = \dfrac{1}{n+1}$

P/s. Đã làm mà còn phải cảm ơn nữa hả bạn :))
 
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