Giải:
\(A=\left(x^3+5x^2-2x+1\right)\left(x-7\right)\)
\(\Leftrightarrow A=x\left(x^3+5x^2-2x+1\right)-7\left(x^3+5x^2-2x+1\right)\)
\(\Leftrightarrow A=x.x^3+x.5x^2-x.2x+x.1-7.x^3-7.5x^2+7.2x-7.1\)
\(\Leftrightarrow A=x^4+5x^3-2x^2+x-7x^3-35x^2+14x-7\)
\(\Leftrightarrow A=x^4-2x^3-37x^2+15x-7\)
Vậy ...