`A= 1/3 + 1/6 + 1/10 + 1/15+...+1/190`
`1/2 A= 1/2(1/3 + 1/6 + 1/10+ 1/15+...+1/190)`
`1/2 A= 1/6 + 1/12 + 1/20 + 1/30 +...+1/380`
`1/2A = 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6+...+1/19.20`
`1/2A = 1/2 - 1/3 +1/3-1/4+1/4-....+1/19-1/20`
`1/2 A = 1/2 - 1/20`
`A= (1/2 - 1/20) : 1/2`
`A= (1/2 - 1/20) . 2`
`A= 1- 1/10 <1`
Vậy `A<1`