`y xx (2/3 + 2/15 + 2/35 + 2/56) = 1`
`y xx (2/{1.3} + 2/{3.5} + 2/{5.7} + 2/56) = 1`
`y xx (1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 2/56) = 1`
`y xx (1 - 1/7 + 2/56) = 1`
`y xx (6/7 + 2/56) = 1`
`y xx 25/28 = 1`
`y = 1 : 25/28`
`y = 28/25`
$\text{Vậy}$ `y = 28/25`