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Question

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

This question was previously asked in
SSC CGL 2025 Tier 2 Paper 1 Question Paper (19-Jan-2026)
The correct answer is

\(18\sqrt{3}\)

To find the value of \(x^3+\dfrac{1}{x^3}\) given that \(x+\dfrac{1}{x}=2\sqrt{3}\), we can utilize algebraic identities. Let's solve this problem step by step.

  1. Start with the identity for cubes: \(x^3 + \dfrac{1}{x^3} = \left(x + \dfrac{1}{x}\right)^3 - 3\left(x + \dfrac{1}{x}\right)\).
  2. Substitute the given value: \(x + \dfrac{1}{x} = 2\sqrt{3}\).
  3. Calculate \((x + \dfrac{1}{x})^3\):
    • \((2\sqrt{3})^3 = 8 \times 3^{3/2} = 8 \times 3\sqrt{3} = 24\sqrt{3}\).
  4. Find \(3 \left(x + \dfrac{1}{x}\right)\):
    • \(3 \times 2\sqrt{3} = 6\sqrt{3}\).
  5. Substitute into the identity: \(x^3 + \dfrac{1}{x^3} = 24\sqrt{3} - 6\sqrt{3}\).
  6. Simplify: \(x^3 + \dfrac{1}{x^3} = 18\sqrt{3}\).

Thus, the value of \(x^3+\dfrac{1}{x^3}\) is \(18\sqrt{3}\), which corresponds to the correct option.

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