ICSEClass XMathematicsFactorisation04 Mark2023 Easy

QQuestion

Find the value of ‘a’ if x − a is a factor of the polynomial 3x3 + x2 − ax − 81.

Exam-ready answer 04 Mark

Answer

Let f(x) = 3x3 + x2 − ax − 81.

By the Factor Theorem, if x − a is a factor of f(x), then f(a) = 0.

Since x − a = 0, x = a.

Substituting x = a:

⇒ 3a3 + a2 − a(a) − 81 = 0

⇒ 3a3 + a2 − a2 − 81 = 0

⇒ 3a3 − 81 = 0

⇒ 3a3 = 81

⇒ a3 = \frac{81}{3}

⇒ a3 = 27

⇒ a3 = 33

Therefore, a = 3.

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