FM November 2022 p11 q01
4246
The cubic equation \(x^3 + bx^2 + d = 0\) has roots \(\alpha, \beta, \gamma\), where \(\alpha = \beta\) and \(d \neq 0\).
(a) Show that \(4b^3 + 27d = 0\).
(b) Given that \(2\alpha^2 + \gamma^2 = 3b\), find the values of \(b\) and \(d\).
Solution
(a) From the roots \(\alpha, \alpha, \gamma\), we have:
\(2\alpha + \gamma = -b\)
\(\alpha^2 + 2\alpha\gamma = 0\)
Solving, \(\alpha = -2\gamma\) leading to \(-4\gamma + \gamma = -b\), so \(\gamma = \frac{1}{3}b\) and \(\alpha = -\frac{2}{3}b\).
\(\alpha^2\gamma = -d\) leading to \(\frac{4}{27}b^3 = -d\) leading to \(4b^3 + 27d = 0\).
(b) Given \(2\alpha^2 + \gamma^2 = 3b\), substitute for \(\alpha\) and \(\gamma\):
\(3b = b^2\) leading to \(b = 3\).
Substitute back to find \(d\):
\(d = -4\).
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