"If x² - 5x + 4 is a factor of x⁴ - px² + q, then what are the values of p and q respectively?"

This question was previously asked in
UPSC CDS-I 2025 (Elementary Mathematics) Official Paper (Held On: 13 Apr, 2025)
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  1. 17, 16
  2. 16, 17
  3. 15, 16
  4. 16, 15

Answer (Detailed Solution Below)

Option 1 : 17, 16
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Detailed Solution

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Given:

If x2 - 5x + 4 is a factor of x4 - px2 + q.

Formula used:

If (x2 - 5x + 4) is a factor of (x4 - px2 + q), then the roots of x2 - 5x + 4 (i.e., 1 and 4) must satisfy x4 - px2 + q = 0.

Calculation:

Roots of x2 - 5x + 4 are x = 1 and x = 4.

Substitute x = 1 in x4 - px2 + q = 0

⇒ 14 - p(12) + q = 0

⇒ 1 - p + q = 0

⇒ q = p - 1 ----- (1)

Substitute x = 4 in x4 - px2 + q = 0

⇒ 44 - p(42) + q = 0

⇒ 256 - 16p + q = 0

⇒ q = 16p - 256 ------ (2)

Equate q values from 1 and 2

⇒ p - 1 = 16p - 256

⇒ 255 = 15p

⇒ p = 17

Substitute p = 17 into q = p - 1

⇒ q = 17 - 1

⇒ q = 16

∴ The values of p and q are 17 and 16 respectively.

The correct answer is option (1).

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