Factorise the polynomial x4 − 10x2 + 22 into product of two quadratic polynomials. 

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RRB NTPC Graduate Level CBT-I Official Paper (Held On: 06 Jun, 2025 Shift 2)
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  1. \(\left(x^2 - 5 + \sqrt{3}\right) \left(x^2 - 5 - \sqrt{3}\right) \)
  2. \(\left(x^2 - 3 + \sqrt{3}\right) \left(x^2 - 3 - \sqrt{3}\right)\)
  3. \(\left(x^2 - 4 + \sqrt{3}\right) \left(x^2 - 4 - \sqrt{3}\right) \)
  4. \(\left(x^2 - 2 + \sqrt{3}\right) \left(x^2 - 2 - \sqrt{3}\right)\)

Answer (Detailed Solution Below)

Option 1 : \(\left(x^2 - 5 + \sqrt{3}\right) \left(x^2 - 5 - \sqrt{3}\right) \)
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Detailed Solution

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Given:

Polynomial: x4 − 10x2 + 22

Formula used:

Quadratic formula: For an equation ax2 + bx + c = 0, x = \(\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\)

Calculations:

Let y = x2. The given polynomial can be written as a quadratic equation in y:

y2 - 10y + 22 = 0

Using the quadratic formula to find the roots for y, where a = 1, b = -10, c = 22:

y = \(\frac{-(-10) \pm \sqrt{(-10)^2 - 4 \times 1 \times 22}}{2 \times 1}\)

⇒ y = \(\frac{10 \pm \sqrt{100 - 88}}{2}\)

⇒ y = \(\frac{10 \pm \sqrt{12}}{2}\)

⇒ y = \(\frac{10 \pm 2\sqrt{3}}{2}\)

⇒ y = 5 \(\pm\) \(\sqrt{3}\)

So, the two roots for y are:

y1 = 5 + \(\sqrt{3}\)

y2 = 5 - \(\sqrt{3}\)

Therefore, the quadratic in y can be factored as:

(y - y1)(y - y2) = (y - (5 + \(\sqrt{3}\)))(y - (5 - \(\sqrt{3}\)))

Substitute back y = x2:

(x2 - (5 + \(\sqrt{3}\)))(x2 - (5 - \(\sqrt{3}\)))

∴ The factorization of the polynomial x4 − 10x2 + 22 into a product of two quadratic polynomials is (x2 - (5 + \(\sqrt{3}\)))(x2 - (5 - \(\sqrt{3}\))).

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