`M= (1/2 -1)( 1/3-1)(1/4-1)....(1/100-1)`
`M= (1/2 - 2/2) ( 1/3 - 3/3) ( 1/4 - 4/4) ....( 1/100- 100/100)`
`M=(-1/2) ( -2/3) (-3/4)....(-99/100)`
`M= ((-1).(-2).(-3)....(-99))/(2.3.4...100)`
`M= -(1.2.3....99)/(2.3.4...100)`
`M= -1/100`
Vậy `M = -1/100`
