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Mathematics

Using factor theorem, show that (x - 2) is a factor of x3 + x2 - 4x - 4. Hence, factorise the polynomial completely.

Factorisation

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Answer

By factor theorem, (x - a) is a factor of f(x), if f(a) = 0.

f(x) = x3 + x2 - 4x - 4

(x - 2) is a factor of f(x), if f(2) = 0

f(2)=(2)3+(2)24(2)4=8+484=0f(2) = (2)^3 + (2)^2 - 4(2) - 4 \\[0.5em] = 8 + 4 - 8 - 4 \\[0.5em] = 0

Hence, (x - 2) is a factor of x3 + x2 - 4x - 4.

Now, factorizing x3 + x2 - 4x - 4,

x2(x+1)4(x+1)(x24)(x+1)(x2)(x+2)(x+1)\Rightarrow x^2(x + 1) - 4(x + 1) \\[0.5em] \Rightarrow (x^2 - 4)(x + 1) \\[0.5em] \Rightarrow (x - 2)(x + 2)(x + 1)

Hence, x3 + x2 - 4x - 4 = (x - 2)(x + 1)(x + 2).

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