Đáp án:
`(x + 5)/3 + (x + 5)/15 + (x + 5)/35 + (x + 5)/63 + (x + 5)/99 = 20/11`
`-> (x + 5) [1/3 + 1/15 + 1/35 + 1/63 + 1/99] = 20/11`
`-> (x + 5) [1/(1 . 3) + 1/(3 . 5) + 1/(5 . 7) + 1/(7 . 9) + 1/(9 . 11)] = 20/11`
`-> (x + 5) [1/2 (1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11)] = 20/11`
`-> (x + 5)[1/2 (1 - 1/11)] = 20/11`
`-> (x + 5)[ 1/2 . 10/11] = 20/11`
`-> (x + 5) . 5/11 = 20/11`
`-> x + 5 = 20/11 : 5/11`
`-> x + 5 = 4`
`-> x = 4 - 5`
`-> x = -1`
`text{Vậy x = -1}`
