Đáp án:
`M = (1 - 2/6) (1 - 2/12) (1 - 2/20) ... (1 - 2/9900)`
`⇔ M = (6/6 - 2/6) (12/12 - 2/12) (20/20 -2/20) ... (9900/9900 - 2/9900)`
`⇔ M = 4/6 . 10/12 . 18/20 ... 9898/9900`
`⇔ M = (1 . 4)/(2 . 3) . (2 . 5)/(3 . 4) . (3 . 6)/(4 . 5) ... (98 . 101)/( 99 . 100)`
`⇔ M = 1/2 . 4/3 . 2/3 . 5/4 . 3/4 . 6/5 ... 98/99 . 101/100`
`⇔ M = (1/2 . 2/3 . 3/4 ... 98/99) . (4/3 . 5/4 . 6/5 ... 101/100)`
`⇔ M = (1 . 2 . 3 ... 98)/(2 . 3 . 4 ... 99) . (4 . 5 . 6 ... 101)/(3 . 4 . 5 ... 100)`
`⇔ M = 1/99 . 101/3`
`⇔ M = (1 . 101)/(99 . 3)`
`⇔ M = 101/297`
Vậy `M = 101/297`