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If x - 1 and x + 1 are both factors of the equation x\(^3\)...

Mathematics
JAMB 1995

If x - 1 and x + 1 are both factors of the equation x\(^3\) + px\(^2\) + qx + 6 = 0, evaluate p and q

  • A. -6, -1
  • B. 6, 1
  • C. 1, -1
  • D. 6, -6
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Correct Answer: Option A
Explanation

\(x^{3} + px^{2} + qx + 6 = 0\)

f(x - 1) = 0; f(1) = 0.

\(1^{3} + p(1^{2}) + q(1) + 6 = 0 \implies p + q = -7 ... (1)\)

f(x + 1) = 0; f(-1) = 0.

\((-1)^{3} + p(-1^{2}) + q(-1) + 6 = 0 \implies p - q = -5 ... (2)\)

Subtract (2) from (1).

\(2q = -2 \implies q = -1\)

\(p - (-1) = -5 \implies p = -5 - 1 = -6\)

\((p, q) = (-6, -1)\)


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