`\text{a)}`
`A = 1/{5.6} +1/{6.7} +1/{7.8} +... +1/{24.25}`
`=> A = 1/5 - 1/6 + 1/6 - 1/7 +1/7 - 1/8 +1/8 + ... +1/24 - 1/25`
`=> A = 1/5 + (-1/6+1/6) + (-1/7+1/7) + (-1/8+1/8) + ... + (-1/24 +1/24) - 1/25`
`=> A = 1/5 - 1/25`
`=> A = 5/25 - 12/5 =4/25`
Vậy `A =4/25`
$\\$
`\text{b)}`
Sửa đề : `28/{57.85}` thay cho `30/{57.85}`
`B = 2/{3.5} +3/{5.8} +11/{8.19} +13/{19. 32} +25/{32.57} +28/{57.85}`
`=> B = 1/3 -1/5 +1/5 -1/8 +1/8 -1/19 +1/19 - 1/32 +1/32 -1/57 +1/57-1/85`
`=> B = 1/3+ (-1/5+1/5) + (-1/8+1/8) + (-1/9+1/9) + (-1/32+1/32) + (-1/57 +1/57) - 1/85`
`=> B = 1/3 -1/85 = 82/255`
Vậy `B = 82/255`
