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If x2 - 3x + 2 is a factor of x3 - ax2 + b, find the values of a and b.

Factorisation

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Answer

Factorising, x2 - 3x + 2.

⇒ x2 - 2x - x + 2

⇒ x(x - 2) - 1(x - 2)

⇒ (x - 1)(x - 2).

So, we can say (x - 1) and (x - 2) are the factors of x3 - ax2 + b.

By factor theorem,

A polynomial f(x) has a factor (x - a), if and only if, f(a) = 0.

Given, 1st factor

⇒ x - 1 = 0

⇒ x = 1.

Substituting x = 1 in x3 - ax2 + b, will give remainder = 0.

⇒ 13 - a(1)2 + b = 0

⇒ 1 - a + b = 0

⇒ a = b + 1 ………(Eq. 1)

Given, 2nd factor

⇒ x - 2 = 0

⇒ x = 2.

Substituting x = 2 in x3 - ax2 + b, will give remainder = 0.

⇒ 23 - a(2)2 + b = 0

⇒ 8 - 4a + b = 0

⇒ 4a = b + 8

Substituting value of a from Eq 1 in above equation :

⇒ 4(b + 1) = b + 8

⇒ 4b + 4 = b + 8

⇒ 4b - b = 8 - 4

⇒ 3b = 4

⇒ b = 43\dfrac{4}{3}

⇒ a = b + 1 = 43+1=4+33=73\dfrac{4}{3} + 1 = \dfrac{4 + 3}{3} = \dfrac{7}{3}.

Hence, a = 43\dfrac{4}{3} and b = 73\dfrac{7}{3}.

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