RÚT GỌN  $B=(1-\frac{1}{2})$ $.(1-\frac{1}{3}).$ $1-\frac{1}{4}…($ $1-\frac{1}{20})$ 

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1. Đáp án:

    

   Giải thích các bước giải:

    $B=$$\frac{1}{2}$.$\frac{2}{3}$.$\frac{3}{4}$….. $\frac{19}{20}$= $\frac{1.2.3…..19}{2.3.4…..20}$= $\frac{1}{20}$ 

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2. `B=(1 – 1/2).(1 – 1/3).(1 – 1/4)…(1 – 1/20)`

   `B=(2/2 – 1/2).(3/3 – 1/3).(4/4 – 1/4)…(20/20 – 1/20)`

   `B=1/2 . 2/3 . 3/4 … 19/20`

   `B=(1.2.3…19)/(2.3.4…20)`

   `B=1/20`

    

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