Theo bài ra ta có phương trình:
\(\left\{{}\begin{matrix}P\left(-1\right)=a.\left(-1\right)^3+b\left(-1\right)^2+c.\left(-1\right)+d=50\\P\left(0\right)=a.0+b.0+c.0+d=1\\P\left(1\right)=a.1^3+b.1^2+c.1+d=100\\P\left(2\right)=a.2^3+b.2^2+c.2+d=120\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}P\left(-1\right)=-a+b-c+d=50\\P\left(0\right)=d=1\\P\left(1\right)=a+b+c+d=100\\P\left(2\right)=8a+4b+2x+d=120\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}P\left(-1\right)=-a+b-c+1=50\\P\left(1\right)=a+b+c+1=100\\P\left(2\right)=8a+4b+2c+1=120\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}P\left(-1\right)=-a+c-c=49\\P\left(1\right)=a+b+c=99\\P\left(2\right)=8a+4b+2c=119\end{matrix}\right.\)
Giải hệ phương trình trên, ta được:
\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{-227}{6}\\b=74\\c=\dfrac{377}{6}\\d=1\end{matrix}\right.\)
Thay \(a=\dfrac{-227}{6},b=74,c=\dfrac{377}{6},d=1\) và \(x=3\) vào đa thức \(P\left(x\right)=ax^3+bx^2+cx+d\) ta được:
\(P\left(3\right)=\left(\dfrac{-227}{6}\right).3^3+74.3^2+\dfrac{377}{6}.3+1\)
\(P\left(3\right)=-166\)
Vậy P(3)=-166
ĐK : \(a\ne0\) .
Theo bài ra ta có hệ phương trình :
\(\left\{{}\begin{matrix}a+b+c+1=100\\-a+b-c+1=50\\d=1\\8a+4b+2c+1=120\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}a+b+c=99\\-a+b-c=49\\8a+4b+2c=119\\d=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{-227}{6}\\b=74\\c=\dfrac{377}{6}\\d=1\end{matrix}\right.\)
\(\Rightarrow P\left(x\right)=-\dfrac{227}{6}x^3+74x^2+\dfrac{377}{6}x+1\)
\(\Rightarrow P\left(3\right)=-\dfrac{227}{6}.3^3+74.3^2+\dfrac{377}{6}.3+1=-166\)