c: Ta có: \(P=\left(2x\right)^2\cdot\left(x^3-x\right)-x^2\left(4x^3+2x-1\right)-\dfrac{1}{2}x\left(4x-12x^2\right)\)
\(=4x^2\left(x^3-x\right)-x^2\left(4x^3+2x-1\right)-\dfrac{1}{2}x\left(4x-12x^2\right)\)
\(=4x^5-4x^3-4x^5-2x^3+x^2-2x^2+6x^3\)
\(=-x^2\)
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