Q.If \(x - \cfrac{1}{x} = \sqrt{5}\), then the value of \(x^4 + \cfrac{1}{x^4}\) is — (a) 45 (b) 46 (c) 47 (d) 48
Answer: C
\(x^2 + \cfrac{1}{x^2} = (x - \cfrac{1}{x})^2 + 2 \cdot x \cdot \cfrac{1}{x}\) \(= (\sqrt{5})^2 + 2 = 5 + 2 = 7\) \(\therefore x^4 + \cfrac{1}{x^4} = (x^2 + \cfrac{1}{x^2})^2 - 2 \cdot x^2 \cdot \cfrac{1}{x^2}\) \(= 7^2 - 2 = 49 - 2 = 47\)
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