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Previous Year Question (PYQs)
4
The values of the parameter $a$ such that the roots $\alpha, \beta$ of
$2x^2 + 6x + a = 0$ satisfy the inequality $\dfrac{\alpha}{\beta} + \dfrac{\beta}{\alpha} < 2$ are —
Solution
For the quadratic $2x^2 + 6x + a = 0$:
$\alpha + \beta = -\dfrac{6}{2} = -3$, and $\alpha\beta = \dfrac{a}{2}$.
Now,
$\dfrac{\alpha}{\beta} + \dfrac{\beta}{\alpha} = \dfrac{\alpha^2 + \beta^2}{\alpha\beta}
= \dfrac{(\alpha + \beta)^2 - 2\alpha\beta}{\alpha\beta}
= \dfrac{9 - 2\cdot \frac{a}{2}}{\frac{a}{2}} = \dfrac{18 - 2a}{a}.$
Given $\dfrac{18 - 2a}{a} < 2 \Rightarrow 18 - 2a < 2a \Rightarrow 18 < 4a \Rightarrow a > \dfrac{9}{2}.$
For real roots, discriminant $36 - 8a \ge 0 \Rightarrow a \le \dfrac{9}{2}.$
Hence, no real $a$ satisfies both.
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