`•` Phân tích:
`2x^2+x-11`
`=2(x^2+x-11/2)`
`=2(x^2+x+1/4-23/4)`
`=2[(x+1/2)^2-23/4]`
`=2(x+1/2-sqrt23/2)(x+1/2+sqrt23/2)`
`•` Giải phương trình:
`2x^2+x-11=0`
`<=>2(x^2+x-11/2)=0`
`<=>2(x^2+x+1/4-23/4)=0`
`<=>2[(x+1/2)^2-23/4]=0`
`<=>2(x+1/2-sqrt23/2)(x+1/2+sqrt23/2)=0`
`<=>(x+1/2-sqrt23/2)(x+1/2+sqrt23/2)=0`
`+)x+1/2-sqrt23/2=0`
`<=>x=(sqrt23-1)/2`
`+)x+1/2+sqrt23/2=0`
`<=>x=(-sqrt23-1)/2`
Vậy `S={(-sqrt23-1)/2,(sqrt23-1)/2}`.
