Ta có:
\(\left(3a-2b+c\right)^2=9a^2+4b^2+c^2+2\left(3ac-6ab-2bc\right)\)
\(\Rightarrow b^2=9a^2+4b^2+c^2\)
(vì \(3a-3b+c=0\Leftrightarrow3a-2b+c=-b\), \(6ab+2bc-3ac=0\))
\(\Leftrightarrow9a^2+3b^2+c^2=0\)
\(\Leftrightarrow a=b=c=0\).
Khi đó: \(P=\left(-1\right)^{2019}+\left(-1\right)^{2020}+\left(-1\right)^{2021}=-1\)
Ta có:
(3a−2b+c)2=9a2+4b2+c2+2(3ac−6ab−2bc)
⇒b2=9a2+4b2+c2
(vì 3a−3b+c=0⇔3a−2b+c=−b, 6ab+2bc−3ac=0)
⇔9a2+3b2+c2=0
⇔a=b=c=0.
Khi đó: P=(−1)2019+(−1)2020+(−1)2021=−1