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第6題
設三次方程式\(x^3+ax^2+bx+8=0\)的實數根為\(\alpha,\beta,\gamma\),且\(\alpha<\beta<\gamma\)。若數列\(\alpha,\beta,\gamma\)成等差數列,且\(\beta,\alpha,\gamma\)成等比數列,則\(b=\) 。
[解答]
\(\begin{align}
& {{\alpha }^{2}}=\beta \gamma \\
& \alpha \beta \gamma =-8 \\
& {{\alpha }^{3}}=-8 \\
& \alpha =-2 \\
& \beta \gamma =4 \\
& \\
& \beta =-2+d,\gamma =-2+2d \\
& \left( -2+d \right)\left( -2+2d \right)=4 \\
& d=3 \\
& \beta =1,\gamma =4 \\
& ...... \\
\end{align}\)