Giải
\(P\left(-1\right)=\left(a-b+c\right)\)
\(P\left(-2\right)=\left(4a-2b+c\right)\)
\(P\left(-1\right)+P\left(-2\right)=\left(a-b+c\right)+\left(4a-2b+c\right)=5a-3b+2c=0\)
\(\Rightarrow\) \(P\left(-1\right)=-P\left(-2\right)\)
Do đó \(P\left(-1\right).P\left(-2\right)\) = \(\left[P\left(-2\right)\right]^2\le0\)