`4x^2 - 5x -2 =0`
`=>4 . (x^2 - 5/4x ) - 2 = 0`
`=> 4. (x^2 - 5/4x+ 25/64) - 57/16 = 0`
`=> 4. (x^2 - 5/8x - 5/8x + 25/64) - 57/16 = 0`
`=> 4. [ x . (x-5/8) - 5/8.(x-5/8) ] - 57/16 = 0`
`=> 4. (x-5/8).(x-5/8) - 57/16 =0`
`=> 4.(x-5/8)^2 - 57/16=0`
`=> 4.(x-5/8)^2 = 57/16`
`=> (x-5/8)^2 = 57/16 :4`
`=> (x-5/8)^2 = 57/64`
`=> (x-5/8)^2 = ((\sqrt{57})/8)^2`
`=> x-5/8= (\sqrt{57})/8 ` hoặc `x-5/8 = (-\sqrt{58})/8`
`+) x- 5/8 = (\sqrt{57})/8`
`=> x = (\sqrt{57})/8 + 5/8`
`=> x = (sqrt{57} + 5)/8`
`+) x-5/8 = (-\sqrt{58})/8`
`=> x = (-\sqrt{58})/8 + 5/8`
`=> x = (-sqrt{57} + 5)/8`
Vậy `x\in{ (sqrt{57} + 5)/8; (-sqrt{57} + 5)/8}`
