9)
`(100-(1+1/2+1/3+...+1/100) : (1/2 +2/3+...+99/100)`
`= (100-1 -1/2 -1/3-...-1/100) : (1/2 +2/3+...+99/100)`
`= [(1-1) + ( 1-1/2) + (1-1/3)+...+(1-1/100)] : (1/2 +2/3+...+99/100)`
`= (1/2+2/3+ ...+99/100):(1/2 +2/3+...+99/100)`
`=1`
10)
`A= 1-1/3 -1/6-...-1/210`
`1/2A = 1/2 -1/6-1/12-...-1/420`
`1- 1/2A = 1-(1/2 +1/6+1/12 +...+1/420)`
`1/2 A= 1-( 1/1.2 +1/2.3 + 1/3.4+...+1/20.21)`
`1/2A= 1- ( 1/1 -1/2 + 1/2-1/3+...+1/20 -1/21)`
`1/2A = 1-( 1 - 1/21)`
`1/2A= 1- 20/21`
`1/2A= 1/21`
`A= 1/21 :1/2`
`A= 2/21`
Vậy `A=2/21`
