If x > 0, and \(x^4 + \frac{1}{x^4} = 254\), what is the value of \(x^5 + \frac{1}{x^5}\)?

This question was previously asked in
SSC CGL 2023 Tier-I Official Paper (Held On: 14 Jul 2023 Shift 4)
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  1. 717√2
  2. 723√2
  3. 720√2
  4. 726√2

Answer (Detailed Solution Below)

Option 1 : 717√2
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Detailed Solution

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Given:

\(x^4 + \frac{1}{x^4} = 254\)

Formula used:

\(x^5 + \frac{1}{x^5} \) = ( \(x^3 + \frac{1}{x^3} \) ) ×  ( \(x^2 + \frac{1}{x^2} \)) - \((x + \frac{1}{x} )\)

Calculation:

\(x^4 + \frac{1}{x^4} +2 = [{x^2 + \frac{1}{x^2}}]^2\)

⇒ \( [{x^2 + \frac{1}{x^2}}]^2\) = 254 + 2

⇒ \( [{x^2 + \frac{1}{x^2}}]\) = 16

⇒ ​\(x^2 + \frac{1}{x^2} +2 = [{x + \frac{1}{x}}]^2 \)

 \((x + \frac{1}{x} )\) = √16+2

 ​\((x + \frac{1}{x} )\) = 3√2

 ​\((x^3 + \frac{1}{x^3} )\) = (3√2)3 - 3 × 3√2

 ​\((x^3 + \frac{1}{x^3} )\) = 54√2 - 9√2

 ​\((x^3 + \frac{1}{x^3} )\) = 45√2

\(x^5 + \frac{1}{x^5} \) = 45√2 × 16 - 3√2

⇒ \(x^5 + \frac{1}{x^5} \) = 717√2

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