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Question

$16(x^4 + \frac{1}{x^4}) - 257 = 0$

What is \(\left(x^3 + \frac{1}{x^3}\right)\) equal to ?

This question was previously asked in
CDS 2 2026 Maths Question Paper (13-Sep-2026)
The correct answer is

\(\frac{65}{8}\)

To find the value of \(x^3 + \frac{1}{x^3}\), we have been given the equation:

\(16\left(x^4 + \frac{1}{x^4}\right) - 257 = 0\)

Let's work through the solution to find the required expression:

  1. Firstly, solve the given equation for \(x^4 + \frac{1}{x^4}\):

\(16(x^4 + \frac{1}{x^4}) = 257\)

  1. Divide both sides by 16 to isolate \(x^4 + \frac{1}{x^4}\):

\(x^4 + \frac{1}{x^4} = \frac{257}{16}\)

  1. Recall a mathematical identity: \((x^2 + \frac{1}{x^2})^2 = x^4 + \frac{1}{x^4} + 2\). Therefore, we can express \(x^4 + \frac{1}{x^4}\) in terms of \(x^2 + \frac{1}{x^2}\):

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

  1. Substitute the value of \(x^4 + \frac{1}{x^4}\):

\((x^2 + \frac{1}{x^2})^2 = \frac{257}{16} + 2 = \frac{289}{16}\)

  1. Taking the square root on both sides, we find:

\(x^2 + \frac{1}{x^2} = \frac{17}{4}\)

  1. Again, recall another identity: \((x + \frac{1}{x})^2 = x^2 + \frac{1}{x^2} + 2\). Use this to find \(x + \frac{1}{x}\):

\((x + \frac{1}{x})^2 = \frac{17}{4} + 2 = \frac{25}{4}\)

  1. Take the square root to get:

\(x + \frac{1}{x} = \frac{5}{2}\)

  1. Now, use the expression: \((x + \frac{1}{x})^3 = x^3 + \frac{1}{x^3} + 3(x + \frac{1}{x})\) to solve for \(x^3 + \frac{1}{x^3}\):

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

  1. Calculate \((\frac{5}{2})^3\) and \(3(\frac{5}{2})\):

\(\frac{125}{8} = x^3 + \frac{1}{x^3} + \frac{15}{2}\)

  1. Convert \(\frac{15}{2}\) to a fraction with denominator 8:

\(\frac{15}{2} = \frac{60}{8}\)

  1. Substitute back to solve for \(x^3 + \frac{1}{x^3}\):

\(\frac{125}{8} = x^3 + \frac{1}{x^3} + \frac{60}{8}\)

  1. Rearrange the terms to find:

\(x^3 + \frac{1}{x^3} = \frac{125}{8} - \frac{60}{8} = \frac{65}{8}\)

Thus, the value of \(x^3 + \frac{1}{x^3}\) is \(\frac{65}{8}\), which corresponds to the correct answer given.

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