Đáp án:
`a) y=1`
`b)y=-5` hoặc `y=1`
Giải thích các bước giải:
` a) (y-2)^3-(y-3)(y^2+3y+9)+6(y+1)^2=49`
`⇔y^3-6y^2+12y-8-y^3+27+6(y^2+2y+1)-49=0`
`⇔y^3-6y^2+12y-8-y^3+27+6y^2+12y+6-49=0`
`⇔24y-24=0`
`⇔y=1`
Vậy `y=1`
`b)(y+3)^3-(y+1)^3=56`
`⇔y^3+9y^2+27y+27-y^3-3y^2-3y-1-56=0`
`⇔6y^2+24y-30=0`
`⇔6(y^2+4y-5)=0`
`⇔y^2+4y-5=0 (`Vì `6>0)`
`⇔y^2-y+5y-5=0`
`⇔y(y-1)+5(y-1)=0`
`⇔(y+5)(y-1)=0`
`⇔`\(\left[ \begin{array}{l}y+5=0\\y-1=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}y=-5\\y=1\end{array} \right.\)
Vậy `y=-5` hoặc `y=1`
