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\(P\left(1\right)=a+b+c=0\)
\(P\left(\frac{c}{a}\right)=\frac{ac^2}{a^2}+\frac{bc}{a}+c=0\)
\(\Leftrightarrow\frac{c^2}{a}+\frac{bc}{a}+\frac{ac}{a}=0\)
\(\Leftrightarrow\frac{c^2+bc+ac}{a}=0\)
\(\Leftrightarrow\frac{c\left(c+b+a\right)}{a}=0\Leftrightarrow\frac{c}{a}\left(a+b+c\right)=0\Leftrightarrow a+b+c=0\)
\(\Rightarrow P\left(\frac{c}{a}\right)=0\Rightarrowđpcm\)
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