Theory of Equations
Solve the following equation: $$x^{3} - 18x = 35$$.
Find the roots $$\alpha, \beta, \gamma$$ of $$x^{3}-11x^{2} +36x - 36 = 0$$ if $$\frac{2}{\beta} = \frac{1}{\alpha} + \frac{1}{\gamma}$$
Given that,
$$\alpha, \beta, \gamma$$ are the roots of $$x^3 - 11x^2 + 36x - 36 = 0$$
Here, $$a = 1, b= -11, c = 36, d = -36$$
$$\alpha + \beta + \gamma = \dfrac{-b}{a} = 11$$
$$\alpha\beta + \beta\gamma + \gamma\alpha = \dfrac{c}{a} = 36$$
$$\alpha \beta \gamma = \dfrac{-d}{a} = 36$$
$$\because \dfrac{2}{\beta} = \dfrac{1}{\alpha} + \dfrac{1}{\gamma}$$
$$\implies \dfrac{2}{\beta} = \dfrac{\alpha + \gamma}{\alpha \gamma}$$
$$\implies 2\alpha \gamma= \alpha \beta + \beta \gamma$$
$$\implies 3\alpha \gamma= \alpha \beta + \beta \gamma + \alpha \gamma = 36$$
$$\implies \alpha \gamma = \dfrac{36}{3} = 12$$....................(i)
Now, $$\beta = \dfrac{\alpha \beta \gamma}{\alpha \gamma} = \dfrac{36}{12} = 3$$
$$\implies \alpha + \gamma = 11 - 3 = 8$$................................(ii)
From (i) & (ii) ,
$$\alpha, \beta$$ are $$6$$ and $$2$$
Hence, $$\alpha = 6$$
$$\beta = 3$$
$$\gamma = 2$$
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