Đặt `A= 3/1.3 + 3/3.5 + 3/5.7+....+ 3/99.101`
`A= 3(1/1.3 + 1/3.5 +1/5.7+...+ 1/99.101)`
`A:3 = 1/1.3 + 1/3.5 + 1/5.7 +...+ 1/99.101`
`A:3.2 = 2(1/1.3 + 1/3.5 + 1/5.7+...+1/99.101)`
`A:3.2 = 2/1.3 + 2/3.5 + 2/5.7 +...+ 2/99.101`
`A:3.2= 1 - 1/3 + 1/3 -1/5 +1/5 -1/7+...+1/99-1/101`
`A:3.2 = 1- 1/101`
`A:3.2 = 100/101`
`A= 100/101 : 2.3`
`A= 150/101`
Vậy `A= 150/101`