`(x+3)^3-x.(3x+1)^2+(2x+1).(4x^2-2x+1)=28`
`<=>(x+3)^3-x.(3x+1)^2+(2x+1).((2x)^2-2x.1+1^2)=28`
`<=>x^3+3.x^2. 3+3.x.3^2+27-x.(9x^2+6x+1)+(2x)^3+1=28`
`<=>x^3+9x^2+27x+27-9x^3-6x^2-x+8x^3+1=28`
`<=>(x^3-9x^3+8x^3)+(9x^2-6x^2)+(27x-x)+28=28`
`<=>3x^2+26x=0`
`<=>x.(3x+26)=0`
`<=>`\(\left[ \begin{array}{l}x=0\\3x+26=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=0\\x=\dfrac{-26}{3}\end{array} \right.\)
Vậy `x=0` hoặc `x=-26/3` là giá trị cần tìm.
[ Áp dụng:] `(a+b)^2=a^2+2ab+b^2`
`(a+b)^3=a^3+3.a^2. b+3.a.b^2+b^3`
`(a+b).(a^2-ab+b^2)=a^3+b^3`
