Ta có: `| x+ 1/3| + |x + 4/3|≥ |x+ 1/3 + x + 4/3|`
Dấu `=` xảy ra khi: `|x+ 1/3 + x + 4/3| = 9/2x`
`2x + 5/3 = 9/2x`
`2x + 5/3 - 5/3 = (9x)/2 - 5/3`
`2x = (9x)/2 - (2(-5/3))/2`
`2x = (9x + 2(-5/3))/2`
`2x = (9x - 10/3)/2`
`2.2x = 2.(9x - 10/3)/2`
`4x = 9x - 10/3`
`4x - 9x = 9x - 10/3 - 9x`
`-5x = -10/3`
`(-5x)/(-5) = (-10/3)/(-5)`
`⇒x = 2/3`