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Nov 2004 p3 q3
1442
The polynomial \(2x^3 + ax^2 - 4\) is denoted by \(p(x)\). It is given that \((x - 2)\) is a factor of \(p(x)\).
(i) Find the value of \(a\).
When \(a\) has this value,
(ii) factorise \(p(x)\),
(iii) solve the inequality \(p(x) > 0\), justifying your answer.
Solution
(i) Since \((x - 2)\) is a factor of \(p(x)\), substituting \(x = 2\) into \(p(x)\) should yield zero: \(2(2)^3 + a(2)^2 - 4 = 0\). Simplifying gives \(16 + 4a - 4 = 0\), leading to \(4a = -12\), so \(a = -3\).
(ii) With \(a = -3\), \(p(x) = 2x^3 - 3x^2 - 4\). Dividing by \((x - 2)\) gives the quadratic factor \(2x^2 + x + 2\). Thus, \(p(x) = (x-2)(2x^2 + x + 2)\).
(iii) To solve \(p(x) > 0\), note that \(2x^2 + x + 2\) has no real roots and is always positive. Therefore, \(p(x) > 0\) when \(x > 2\).