`A=(2x-1)(x-1)+2280`
`=2x^2-2x-x+1+2280`
`=2x^2-3x+2281`
`=2x^2-3x+9/8+18239/8`
`=2(x^2-3/2x+9/16)+18239/8`
`=2(x-3/4)^2+18239/8`
Vì `2(x-3/4)^2>=0` với `∀\ x\inRR`
`=>A=2(x-3/4)^2+18239/8>=18239/8`
Dấu `=` xảy ra `<=>x-3/4=0`
`<=>x=3/4`
Vậy `A_{min}=18239/8<=>x=3/4`