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Question

If $x - \frac{1}{x} = 5$, find the value of $x^4 + \frac{1}{x^4}$.

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
727

Solving for $x^4 + \frac{1}{x^4}$

We are given the equation $x - \frac{1}{x} = 5$. We need to find the value of $x^4 + \frac{1}{x^4}$.

Step 1: Find $x^2 + \frac{1}{x^2}$

Square both sides of the given equation:

$ \left(x - \frac{1}{x}\right)^2 = 5^2 $

Expand the left side using the formula $(a-b)^2 = a^2 - 2ab + b^2$:

$ x^2 - 2(x)\left(\frac{1}{x}\right) + \left(\frac{1}{x}\right)^2 = 25 $

Simplify the equation:

$ x^2 - 2 + \frac{1}{x^2} = 25 $

Isolate $x^2 + \frac{1}{x^2}$:

$ x^2 + \frac{1}{x^2} = 25 + 2 $

$ x^2 + \frac{1}{x^2} = 27 $

Step 2: Find $x^4 + \frac{1}{x^4}$

Square both sides of the equation obtained in Step 1:

$ \left(x^2 + \frac{1}{x^2}\right)^2 = 27^2 $

Expand the left side using the formula $(a+b)^2 = a^2 + 2ab + b^2$:

$ (x^2)^2 + 2(x^2)\left(\frac{1}{x^2}\right) + \left(\frac{1}{x^2}\right)^2 = 729 $

Simplify the equation:

$ x^4 + 2 + \frac{1}{x^4} = 729 $

Isolate $x^4 + \frac{1}{x^4}$:

$ x^4 + \frac{1}{x^4} = 729 - 2 $

$ x^4 + \frac{1}{x^4} = 727 $

Final Answer Calculation

The value of $x^4 + \frac{1}{x^4}$ is 727.

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