`A = 1/{2.3} + 1/{3.4} +1/{4.5} + ... + 1/{19.20}`
`=> A = 1/2 -1/3 +1/3 -1/4 +1/4 -1/5 + ... +1/19 -1/20`
`=> A =1/2 - 1/20`
`=> A = 10/20 -1/20`
`=> A = 9/20`
Vậy `A = 9/20`
$\\$
`B =1/7 +1/91 +1/247 +1/475 +1/775 +1/1147`
`=> B = 1/{1.7} + 1/{7.13} + 1/{13 . 19} + 1/{19 . 25} + 1/{25 . 31} +1/{31.37}`
`=> 6B = 6/{1.7} + 6/{7.13} + 6/{13.19}+ 6/{19.25} + 6/{25.31} + 6/{31.37}`
`=> 6B = 1 -1/7 + 1/7 -1/13 +1/13 -1/19 +1/19 - 1/25+1/25 -1/31 + 1/31 -1/37`
`=> 6B = 1 - 1/37 = 36/37`
`=> B = 36/37 . 1/6 = 6/37`
Vậy `B =6/37`
