`\qquad (x+3)(x^2-3x+9)-(x^3-9x+12)=15`
`<=> x^3+27-x^3+9x-12=15`
`<=> 9x+15=15`
`<=> 9x=0`
`<=> x=0`
Vậy `S={0}`
--------------------------------------------------
`\qquad (2x-1)^2+(x+3)^2-5(x+7)(x-7)=0`
`<=> 4x^2-4x+1+x^2+6x+9-5(x^2-49)=0`
`<=> 5x^2+2x+10-5x^2+245=0`
`<=> 2x+255=0`
`<=> 2x=-255`
`<=> x=-255/2`
Vậy `S={-265/2}`
---------------------------------------------------
`\qquad 25x^2-10x+11=0`
`<=> (5x)^2-2.5x.1+1+10=0`
`<=> (5x-1)^2+10=0 (\text{vô lý})`
Vậy `S=∅`
---------------------------------------------------
`\qquad (x-2)^3-(x-3)(x^2+3x+9)+6(x+1)^2=49`
`<=> x^3-6x^2+12x-8-(x^3-27)+6(x^2+2x+1)=49`
`<=> x^3-6x^2+12x-8-x^3+27+6x^2+12x+6=49`
`<=> 24x+25=49`
`<=> 24x=24`
`<=> x=1`
Vậy `S={1}`