$\text{Bài 5:}$
`B=(3x-1)^2-(x+7)^2-2(2x-5)(2x+5)`
`B=9x^2-6x+1-x^2-14x-49-8x^2+50`
`B=-20x+2`
$\text{Thay x = $\dfrac{1}{5}$}$
`⇒B=-20 . 1/5 + 2 = -4 + 2 = -2`
$\text{Bài 8:}$
`a,(x+1)^3+(x-2)^3-2x^2(x-1,5)=3`
`⇔x^3+3x^2+3x+1+x^3-6x^2+12x-8-2x^3+3x^2=3`
`⇔15x=10`
`⇔x=2/3`
`b,(x+2)(x^2-2x+4)(x-2)(x^2+2x+4)=-65`
`⇔[(x+2)(x^2-2x+4)][(x-2)(x^2+2x+4)]=-65`
`⇔(x^3+8)(x^3-8)=-65`
`⇔x^6 - 64 = -65`
`⇔x^6 = -1`
`⇔x∈∅`
