Tính B = ( 1 – $\frac{2}{42}$ ) ( 1 – $\frac{2}{56}$ ) ……. ( 1 – $\frac{2}{9900}$ )

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1. `(1-2/42)(1-2/56)…(1-2/9900)`

   `= (40)/42.(54)/56…9898/9900`

   `= (5.8)/(6.9) . (6.9)/(7.8). (98.101)/(99.100)`

   `= (5.6…98)/(6.7…99).(8.9…101)/(7.8…100)`

   `= (5)/59.(101)/7`

   `= 505/693`

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2. `(1-2/42)(1-2/56)…(1-2/9900)`

   `=40/42xx54/56xx…xx9898/9900`

   `=(5xx8)/(6xx9)xx(6xx9)/(7xx8)xx…xx(98xx101)/(99xx100)`

   `=(5xx6xx7xx8xx…xx98)/(6xx7xx8xx…xx99)xx(8xx9xx10xx…101)/(7xx8xx9xx100)`

   `=5/99xx101/7`

   `=505/693`

    

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