~ Bạn tham khảo ~
`a, A = 1/[1xx2] + 1/[2xx3] + 1/[3xx4] + ... + 1/[9xx10]`
`=> A = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10`
`=> A = 1 - 1/10`
`=> A = 9/10`
`b, B = 2/[1xx3] + 2/[3xx5] + 2/[5xx7] + ... + 2/[19xx21]`
`=> B = 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/19 - 1/21`
`=> B = 1 - 1/21`
`=> B = 20/21`
`c, C=1/[1xx4] + 1/[4xx7] + 1/[7 xx 10] + ... + 1/[34xx37]`
`=> 3C = 3/[1xx4] + 3/[4xx7] + 3/[7xx10] + ... + 3/[34xx37]`
`=> 3C = 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/34 - 1/37`
`=> 3C = 1 - 1/37`
`=> 3C = 36/37`
`=> C = 36/37 ÷ 3`
`=> C = 12/37`
