Đáp án:
`1, F(x)=x^4+2x^3+2x^2-2x-3`
`F(x)=x^4-x^3+3x^3-3x^2+5x^2-5x+3x-3`
`F(x)=x^3(x-1)+3x^2(x-1)+5x(x-1)+3(x-1)`
`F(x)=(x-1)(x^3+3x^2+5x+3)`
`F(x)=(x-1)(x^3+x^2+2x^2+2x+3x+3)`
`F(x)=(x-1)[x^2(x+1)+2x(x+1)+3(x+1)]`
`F(x)=(x-1)(x+1)(x^2+2x+3)`
`=>+-1` là nghiệm của `F(x)`
`2, G=5x^2-4x+6xy+9y^2+5`
`G=(4x^2-4x+1)+(x^2+6xy+9y^2)+4`
`G=(2x-1)^2+(x+3y)^2+4>=4`
dấu = có khi `2x-1=0; x+3y=0<=>x=1/2; x=-3y`
`<=> x=1/2; y=-1/6`
vậy `min G=4` khi `x=1/2; y=-1/6`
