If \(\frac{{{{\rm{x}}^2} + 1}}{{\rm{x}}} = 4\frac{1}{4}\), then what is the value of \({{\rm{x}}^3} + \frac{1}{{{{\rm{x}}^3}}}?\)

This question was previously asked in
SSC CPO Tier- I Previous Paper 14 (Held on: 7th July 2017 Shift 2) 
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  1. 529/16
  2. 527/64
  3. 4913/64
  4. 4097/64

Answer (Detailed Solution Below)

Option 4 : 4097/64
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SSC CPO : General Intelligence & Reasoning Sectional Test 1
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Detailed Solution

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Calculation:

Given that,

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

⇒ x + 1/x = 17/4     ----(1)

Cubing both sides

(x + 1/x)3 = (17/4)3

We know that,

(a + b)3 = a3 + b3 + 3ab(a + b)

From equation (1),

⇒ x3 + 1/x3 + 3x(1/x)(x + 1/x) = (17/4)3

⇒ x3 + 1/x3 + 3x + 3/x = (17/4)3

⇒ (x3 + 1/x3) + 3(x + 1/x) = 4913/64

⇒ (x3 + 1/x3) = 4913/64 - 3 × (17/4)

⇒ (x3 + 1/x3) = 4097/64

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