$\text{Bài 2:}$
`a) x - 20/11.13 - 20/13.15 - .... - 20/53.55 = 3/11`
`⇒ x - (20/11.13 + 20/13.15 +...+ 20/53.55)=3/11`
`⇒ x - 10.(2/11.13 + 2/13.15 +...+ 2/53.55)=3/11`
`⇒ x - 10.(1/11 - 1/13 + 1/13 - 1/15 +...+ 1/53 - 1/55)=3/11`
`⇒ x - 10.(1/11 - 1/55)=3/11`
`⇒ x - 10 . 4/55 = 3/11`
`⇒ x - 8/11 = 3/11`
`⇒ x = 3/11 + 8/11`
`⇒ x = 1`
`b) 1/5.8 + 1/8.11 +...+ 1/(x.(x+3)) = 101/1540`
`⇒ 1/3 . (3/5.8 + 3/8.11 +...+ 3/(x.(x+3))) = 101/1540`
`⇒ 1/3 .(1/5 - 1/8 + 1/8 - 1/11 +...+ 1/x - 1/(x+3)) = 101/1540`
`⇒ 1/3 . (1/5 - 1/(x+3)) = 101/1540`
`⇒ 1/15 - 1/(3(x+3)) = 101/1540`
`⇒ 1/(3(x+3)) = 1/15 - 101/1540`
`⇒ 1/(3(x+3)) = 1/924`
`⇒ 3(x+3) = 924`
`⇒ x + 3 = 308`
`⇒ x = 305`
