`Đặt:`
`B = 1.98 + 2.97 + ... + 98.1`
`1.98 = 1 + 1 + ... + 1 (98 số)`
`2.97 = 2 + 2 + ... + 2 (97 số)`
`⇒ 1.98 + 2.97 + ... + 98.1`
`= 1 + (1 + 2) + (1 + 2 + 3) + ... + (1 + 2 + ... 98)`
`Có: 1 + 2 + ... + n = ((n + 1)n)/2`
`⇒ B = (1.2)/2 + (2.3)/2 + (3.4)/2 + ... + (98.99)/2`
`= 1/2 .(1.2 + 2.3 + 3.4 + ... + 98.99)`
`⇒ A = (1.98 + 2.97 + ... + 98.1)/(1.2 + 2.3 + ... + 98.99)`
`A = (1/2 .(1.2 + 2.3 + 3.4 + ... + 98.99))/(1.2 + 2.3 + ... + 98.99)`
`A = 1/2`
