Toán Lớp 8: ( y + 3 )^3 – ( y + 1 )^3 = 56
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Toán Lớp 8: ( y + 3 )^3 – ( y + 1 )^3 = 56
Comments ( 2 )
$⇔(y+3-y-1)[(y+3)^2+(y+3)(y+1)+(y+1)^2]=56^{}$
$⇔2(y^2+6y+9+y^2+4y+3+y^2+2y+1)=56^{}$
$⇔3y^2+12y+13=28^{}$
$⇔3y^+12y-15=0^{}$
$⇔(3y^2+15y)-(3y+15)=0^{}$
$⇔3y(y+5)-3(y+5)=0^{}$
$⇔3(y-1)(y+5)=0^{}$
$⇔\left[\begin{matrix} x-1=0\\ x+5=0\end{matrix}\right.$
$⇔\left[\begin{matrix} x=1\\ x=5\end{matrix}\right.$