`(x-20)/10 + (x-19)/11 = (x-18)/12 + (x-17)/13`
`=> ((x-20)/10 - 1) + ((x-19)/11 - 1) = ((x-18)/12 - 1) + ((x-17)/13 - 1)`
`=> (x-20-10)/10 + (x-19-11)/11 = (x-18-12)/12 + (x-17-13)/13 `
`=> (x-20-10)/10 + (x-19-11)/11 - (x-18-12)/12 - (x-17-13)/13 = 0`
`=> (x-30)/10 + (x-30)/11 - (x-30)/12 - (x-30)/13 = 0`
`=> (x - 30) . (1/10 + 1/11 - 1/12 - 1/13) = 0`
mà `1/10 + 1/11 - 1/12 - 1/13 \ne 0`
`=> x - 30 = 0`
`=> x = 30`
Vậy `x = 30` thì `(x-20)/10 + (x-19)/11 = (x-18)/12 + (x-17)/13`
(Chúc bạn học tốt)
