Q.If \(x = \sqrt{3} + \sqrt{2}\), find the value of \(x^3 + \cfrac{1}{x^3}\).

Let \(x = \sqrt{3} + \sqrt{2}\) \(\therefore \cfrac{1}{x} = \cfrac{1}{\sqrt{3} + \sqrt{2}}\) \(= \cfrac{\sqrt{3} - \sqrt{2}}{(\sqrt{3} + \sqrt{2})(\sqrt{3} - \sqrt{2})}\) \(= \cfrac{\sqrt{3} - \sqrt{2}}{3 - 2}\) \(= \sqrt{3} - \sqrt{2}\) \(\therefore x + \cfrac{1}{x} = \sqrt{3} + \sqrt{2} + \sqrt{3} - \sqrt{2} = 2\sqrt{3}\) Now, \[ x^3 + \cfrac{1}{x^3} = \left(x + \cfrac{1}{x}\right)^3 - 3x \cdot \cfrac{1}{x} \cdot \left(x + \cfrac{1}{x}\right) \] \[ = (2\sqrt{3})^3 - 3 \cdot 2\sqrt{3} \] \[ = 24\sqrt{3} - 6\sqrt{3} \] \[ = 18\sqrt{3} \]
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