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Feb/Mar 2021 p32 q2
1413
The polynomial \(ax^3 + 5x^2 - 4x + b\), where \(a\) and \(b\) are constants, is denoted by \(p(x)\). It is given that \((x + 2)\) is a factor of \(p(x)\) and that when \(p(x)\) is divided by \((x + 1)\) the remainder is 2.
Find the values of \(a\) and \(b\).
Solution
Since \((x + 2)\) is a factor of \(p(x)\), we have \(p(-2) = 0\).
Substitute \(x = -2\) into \(p(x)\):
\(a(-2)^3 + 5(-2)^2 - 4(-2) + b = 0\)
\(-8a + 20 + 8 + b = 0\)
\(-8a + b + 28 = 0\)
\(-8a + b = -28\) \(\text{(Equation 1)}\)
Since the remainder when \(p(x)\) is divided by \((x + 1)\) is 2, we have \(p(-1) = 2\).