*Lời giải :
`1/(x (x + 1) ) + 1/( (x + 1) (x + 2) ) + 1/( (x + 2) (x + 3) ) + 1/( (x + 3) (x + 4) ) = 1`
`⇔ 1/x - 1/(x + 1) + 1/(x + 1) - 1/(x + 2) + 1/(x + 2) - 1/(x + 3) + 1/(x + 3) - 1/(x + 4) = 1`
`⇔ 1/x + (- 1/(x + 1) + 1/(x + 1) - 1/(x + 2) + 1/(x + 2) - 1/(x + 3) + 1/(x + 3) ) - 1/(x + 4) = 1`
`⇔ 1/x - 1/(x + 4) =1`
`⇔ 1/(x + 4) = (1 - x)/x`
`⇔ x = (x + 4) (1 - x)`
`⇔ (x + 2)^2 = 8`
`⇔ x +2 = \sqrt{8}` hoặc `x + 2 = - \sqrt{8}`
`-> x= -2 + 2 \sqrt{2}` hoặc `x = -2 - 2 \sqrt{2}`
