Lời giải:
`A=(1-1/21)(1-1/28)....(1-1/1326)`
`=20/21. 27/28. ....1325/1326`
`=40/42 . 54/56....2650/2652`
`=(5.8)/(6.7).(6.9)/(7.8)....(50.53)/(51.52)`
`=(5.8.6.9...50.53)/(6.7.7.8...51.52)`
`=(5.6.7...50)/(6.7.8...51) . (8.9...52)/(7.8...52)`
`=5/51. 53/7`
`=265/357`