\(\left(x+3\right)^3-x\left(3x-1\right)^2+\left(2x+1\right)\left(4x^2-2x+1\right)=28\)
\(\Leftrightarrow x^3+9x^2+27x+27-x\left(9x^2-6x+1\right)+8x^3-4x^2+2x+4x^2-2x+1=28\)
\(\Leftrightarrow15x^2+26x=0\)
\(\Rightarrow x\left(15x+26\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=0\\15x+26=0\end{matrix}\right.\)\(\Rightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{-26}{15}\end{matrix}\right.\)
Vậy..
2.
\(\left(a+b+c\right)^2+\left(b+c-a\right)^2+\left(c+a-b\right)+\left(a+b-c\right)^2\)
\(=a^2+b^2+c^2+2ab+2bc+2ac+b^2+c^2+a^2+2bc-2ac-2ab+c+a-b+a^2+b^2+c^2+2ab-2bc-2ac\)
\(=3a^2+3b^2+3c^2+2ab+2bc-2ac+a-b+c\)
\(=\left(3a^2+a\right)+\left(3b^2+2ab-b\right)+\left(2c^2+2bc-2ac+c\right)\)
\(=a\left(3a+1\right)+b\left(3b+2a-1\right)+c\left(2c+2b-2a+1\right)\)
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